He became the first rookie to reach those totals since Tim Duncan in 1998. On 3 December 2015, he was named Eastern Conference Rookie of the Month for games played in October and November. He ranked third among all rookies in scoring (13.7 ppg) and was second in rebounding (9.3 rpg) and blocked shots (1.89 bpg) for October and November. He went on to claim Eastern Conference Rookie of the Month honours for December and January as well. On 12 February 2016, he scored 30 points for Team World in the Rising Stars Challenge. On 23 March 2016, he tied a career high with 29 points and grabbed 10 rebounds in a 115–107 win over the Chicago Bulls. He was one point short of becoming the first rookie since Patrick Ewing to put up 30 points and 10 rebounds in one game for the Knicks.

Porziņģis followed his parents’ footsteps and started to play basketball at age 6. Once he turned 12, his older brother, Jānis, who played professionally in Europe, would take him to offseason training sessions. He played in youth competition with BK Liepājas Lauvas, the most famous club based in hometown of Liepāja, until he was 15 years old. An agent from Latvia sent video of him around this time to teams in Spain and Italy. In 2010, Baloncesto Sevilla, a club that had a professional team competing in the Liga ACB in Spain, called Porziņģis for a tryout in an attempt to recruit foreign talent to its junior squads. He stood 6 ft 8 in (2.03 m) and weighed 157 lb (71 kg) at this time. Porziņģis recalled the moment and said, “I came here with my brother for two or three days, but it was really hot and I couldn’t play at my best because of that. Still, I received a contract in summer 2010 and I signed it.” Inmaculada Avivar, a nutritionist for Sevilla, diagnosed that Porziņģis was suffering from anemia, meaning that he had fewer red blood cells. This caused him to feel fatigue, shortness of breath, and inability to exercise. Nevertheless, he gradually overcame the condition and began seeing improvement in his game. In addition to his medical problems, Porziņģis had trouble picking up the language in Seville, making him reluctant to return there after his first tryout. In his first season playing for the junior squad, he struggled to communicate with the coaching staff and his teammates and was often sleepy due to his health.

Porziņģis debuted for the youth squad on 4 January 2012, against its counterpart from Barcelona, adding 12 points and 10 rebounds. The opposing side was led by Alexander Zhigulin, who would go on to enter the 2015 NBA draft and withdraw. However, Sevilla was defeated, 56–75. This game was part of the Ciutat de L’Hospitalet tournament, which led up to the Nike International Junior Tournament (NIJT). On 5 January, he scored a personal best of 16 points on the Spars Sarajevo youth squad, helping Sevilla beat their opponents by a margin of 8 points. Porziņģis said, “I knew it was a prestigious tournament and that I had to do well”, he said. “I think I could have done much better, but I wasn’t physically 100 percent. I have seen videos and could have been much more aggressive. But I wish I could have played better.” He finished the tournament averaging 9.2 points, 4.8 rebounds and 2.6 blocks. Porziņģis represented the Sevilla junior team again at the same tournament in early 2013. Against Union Olimpija on 4 January, he recorded 15 points and 6 rebounds. On 6 January, in the team’s tournament finale vs Real Madrid’s youth squad, he scored 24 points and grabbed 11 rebounds. He also made a total of five three-pointers. Porziņģis proved to be more effective in his second year, averaging 16.6 points, 8.4 rebounds and 2.6 blocks and going .481 on three-pointers. This would be his final stint in the youth categories of Sevilla.

Entering the 2012–13 ACB season, Sevilla had hired Aíto García Reneses, who previously worked with the likes of Pau Gasol, Juan Carlos Navarro, Ricky Rubio and Rudy Fernández. On 29 September 2012, Porziņģis got the opportunity to make an appearance with the club’s first team, but was allowed to play just a minute. Sevilla took a tremendous loss to CB Murcia. Following a return to the youth categories of the club at the Torneig de Bàsquet Junior Ciutat de L’Hospitalet, he went back to competing for the senior squad. On 16 January 2013, he logged only four minutes against Spartak Saint Petersburg in the EuroCup, scoring no points, but contributing one rebound and one assist. He scored his first basket for the senior team on 20 February 2013, in a rematch with Spartak. Porziņģis said, “I was very nervous at the beginning: I wanted to do well and not pick up turnovers. At the same time, I got a lot of confidence with players of my same age. That allowed me to play better with the first team and practice with more confidence.” On 4 May 2013, he posted a season-high 7 points with the senior team, hitting two of three shots vs Bilbao Basket in ACB competition.

Jānis Porziņģis competed at the European 2nd tier EuroCup level, the same level in Europe that Kristaps later played at, in one game, and played European professional club basketball in various national leagues, including the Italian League, for more than 10 years. He is known to mentor his younger brother on and off the court and often called him after playing games for Cajasol Sevilla in Spain. Kristaps talked about the relationship in an interview, “We’d break down the details. We watched the film together. He’s always pushing me to work hard. We just spend a lot of time together and we just talk about basketball all the time…” The elder Porziņģis helped him train in the summer and work out in the gym in preparation for international competition in 2012.

Porziņģis opened his second season on Sevilla’s main squad strong in his third game against the EuroLeague team Laboral Kutxa, leading them to a 20-point ACB victory on 2 November 2013. He notched a career-high 12 points, 6 rebounds, and 4 blocks. He broke his scoring record once more against Real Madrid on 30 November 2013, nearly leading his team to a victory. Against the very same team on 6 April 2014, Porziņģis scored a career-high 20 points, nailing 2 three-pointers. Despite his team losing, he began to trend on social media because of his performance. He said, “It was a very good game for me on offense. I didn’t get any rebounds in that game. I had a good shooting night, but could have helped more on defense. Still, fans in Madrid gave me a nice ovation when I fouled out and I liked that a lot.” On 25 May, he got the chance to face his idol Justin Doellman of Barcelona, whom he called the “best power forward in Spanish basketball”. He scored 14 points against Doellman’s team. In May 2014, Porziņģis was selected in the ACB All-Young Players Team of the 2013–14 season.

Porziņģis played with the Latvian youth team and was selected in the 2013 FIBA Europe Under-18 Championship All-Tournament Team.

Prior to the 2014–15 ACB season, head coach Aíto García Reneses parted ways with Sevilla. On 4 October 2014, against CB 1939 Canarias, Porziņģis made his season debut in the Liga ACB, scoring 3 points in the game. He made his first appearance at the 2014–15 EuroCup on 15 October, vs. EWE Baskets Oldenburg and scored 2 points. However, he had a strong EuroCup performance against Pallacanestro Virtus Roma in the weeks that followed, in which he contributed 18 points, 7 rebounds, 4 assists, 4 steals and 2 blocks. He recorded a double-double in mid-November in a win over Estudiantes, adding 11 points and 11 rebounds. On 11 February 2015, he scored a season-high 19 points against the EuroCup’s Turów Zgorzelec. He tied this record on 18 April 2015, vs. Bàsquet Manresa in the ACB. On 15 April 2015, Porziņģis was named the winner of the EuroCup Rising Star Award of the season. One month later, he repeated in the ACB All-Young Players Team, after avoiding the relegation with Baloncesto Sevilla.

In April, Porziņģis declared himself eligible for the 2014 NBA draft. Before and after he made the decision, he drew interest from National Basketball Association (NBA) teams such as the Orlando Magic, who held the 12th overall pick, and traveled to Europe to scout him. The Oklahoma City Thunder reportedly were certainly going to select him with the 21st pick in the case that he remained. Porziņģis was considered an unfinished product and a top-15 draft pick. According to DraftExpress.com, he was the fourth-youngest prospect in their top-100 rankings. Shortly before the day of the draft, however, he withdrew his name. Porziņģis’s agent Andy Miller released the information to ESPN, saying that his client did not feel prepared to become a part of the NBA and wanted to develop his skills until the 2015 draft. One of the league’s general managers commented on him, “He’s very talented. He wasn’t ready, but we would’ve seriously considered drafting him anyway. If he continues to develop his game, get more minutes and his body develops, I think he could be a top-five pick in 2015. He has that kind of talent.”

On 16 April 2015, Porziņģis entered the 2015 NBA draft, according to a report from agent Andy Miller. After gaining a season more of experience, he became known as a lottery pick and a potential top-5 selection. He drew interest from teams such as the Los Angeles Lakers, who had the No. 2 pick and had several personnel that had seen Porziņģis play in Spain. He was compared with players such as Pau Gasol and Dirk Nowitzki, but also Darko Miličić, a former lottery selection widely considered a draft bust. Adrian Wojnarowski of Yahoo! Sports wrote, “Porziņģis has an innate awareness about the way the American public sees a young, long European teenager. He comes to the NBA with the full understanding that popular basketball culture declares him guilty until proven innocent of the basketball crimes of Darko Miličić and Nikoloz Tskitishvili and Andrea Bargnani. He’s considered a stiff, a bust, a blown lottery pick until he doesn’t become one…”

On 25 June 2015, Porziņģis was selected with the fourth overall pick in the 2015 NBA draft by the New York Knicks. He was booed by some New York fans upon being drafted, but vowed to change the fans’ opinions on him from negative to positive. On the same night, the New York Knicks traded for Porziņģis’ teammate from Sevilla, Willy Hernangómez, who was originally drafted by the Philadelphia 76ers as a 35th overall pick. Porziņģis became the highest drafted Latvian and Baltic player in NBA history.

On 30 July 2015, Porziņģis signed his rookie-scale contract with the Knicks. On 28 October 2015, he scored 16 points in his NBA debut, leading New York to a 122–97 season-opening win over the Milwaukee Bucks. He was 3 of 11 from the field and 9 of 12 from the line. On 21 November 2015, he had 24 points, 14 rebounds and seven blocked shots to lead the Knicks to a 107–102 victory over the Houston Rockets, becoming the first 20-year-old to post such a stat line in a single game since Shaquille O’Neal in 1992–93.

In 2015, Porziņģis was named the Latvian Rising Star of the Year.

On 16 November 2016, Porziņģis scored a career-high 35 points in a 105–102 win over the Detroit Pistons. On 11 December 2016, he recorded 26 points, 12 rebounds and a career high-tying seven blocks in a 118–112 win over the Los Angeles Lakers. On 19 January 2017, he came off the bench for the first time in his NBA career after returning from a four-game absence due to a sore left Achilles tendon. He subsequently scored 15 points in a 113–110 loss to the Washington Wizards. During the 2017 All-Star Weekend, Porziņģis played for Team World in the Rising Stars Challenge and won the Skills Challenge.

Following two seasons in Spain and almost two years living in Seville, Porziņģis was able to speak Spanish far more fluently. Porziņģis is thus fluent in three different languages, including English. An NBA executive said, “He speaks great English and I don’t see it being that difficult of a transition off the court.” In October 2016, Porziņģis signed a shoe deal with Adidas, the most lucrative deal for a European player. He made the switch to Adidas after partnering with Nike for his rookie season.

In the Knicks’ season opener the following season, on 19 October 2017, Porziņģis had 31 points and 12 rebounds in a 105–84 loss to the Oklahoma City Thunder. On 30 October 2017, he scored a career-high 38 points in a 116–110 win over the Denver Nuggets. Less than a week later, on 5 November, Porziņģis set a new career high with 40 points in a 108–101 win over the Indiana Pacers. Porziņģis was subsequently named Eastern Conference Player of the Week for games played from Monday, 30 October through Sunday, 5 November. Porziņģis would record the best start to a season for Knicks player, earning 300 points through 10 games, two more than Bernard King had to open the 1984–85 season, and his performances continued, where, on 23 January 2018, he was named an Eastern Conference All-Star reserve.

In 2017, Porziņģis played for Latvia at EuroBasket 2017, where he averaged 23.6 points per game (ranked 3rd), 5.9 rebounds per game (ranked 19th) and 1.9 blocks per game (ranked 1st). Latvia was eliminated in the quarterfinals by Slovenia, losing 103–97.

Porziņģis was born to parents who had experience playing the game of basketball. Tālis, his father, competed semi-professionally before becoming a bus driver. His mother, Ingrīda, was previously on Latvia women’s youth national basketball team. Kristaps’s older brother, Jānis, also played professionally, while Mārtiņš, who is approximately fifteen years older than his youngest sibling, was also an avid player. In a 2017 E:60 documentary on Porziņģis’ life, his parents revealed that they had another son, Toms, who was born four years before Kristaps and died at 14 months. Through an interpreter, Ingrīda said about Toms’ death, “It felt like a bulldozer had run over my life. We had two other children who we had to care for. We had to live on. After Kristaps was born, it was like he had to live for two lives.”

However, on 6 February 2018, in a 103–89 loss to the Milwaukee Bucks, Porziņģis tore his left ACL. He was subsequently ruled out for the rest of the season.

In October 2018, the Knicks decided against signing Porziņģis to a rookie extension, a move that would give New York an extra $10 million in cap space in the summer of 2019 while making Porziņģis a restricted free agent in the offseason. Due to recovering from his ACL injury, Porziņģis did not play for the Knicks to begin the 2018–19 season.

Porziņģis has been compared to Dirk Nowitzki, with the pair having both been groomed in Europe before entering the NBA. Both Porziņģis and Nowitzki are seven-foot-plus players who are comfortable anywhere on the front line and can both shoot from the outside. During his rookie season, Kevin Durant dubbed Porziņģis a basketball “unicorn” because of his rare combination of talents. In January 2018, Porziņģis was averaging 19 shot attempts per game for the Knicks, the most ever by a player his size (the only players 7 ft 3 in (2.21 m) or taller to average 15 field goal attempts per game were Ralph Sampson and Yao Ming). Due to his height and mobility, Porziņģis is able to shoot over any defender, with his sheer size and high volume creating a unique advantage shared by no other player in the league.

On 31 January 2019, after a meeting with the Knicks left franchise officials with the impression that he wanted to be traded, Porziņģis was traded alongside Trey Burke, Courtney Lee and Tim Hardaway Jr. to the Dallas Mavericks in exchange for DeAndre Jordan, Wesley Matthews, Dennis Smith Jr., an unprotected 2021 first-round draft pick, and an additional top-ten protected 2023 first round draft pick. He sat out the remainder of the 2018–19 season, healing from his ACL injury. On 12 July, Porziņģis agreed to re-sign with the Mavericks on a five-year maximum contract worth $158 million.

Porziņģis debuted for the Mavericks on 23 October 2019, posting 23 points and 4 rebounds in a 108–100 win over the Washington Wizards. On 31 January 2020, he scored a then season-high 35 points, along with 12 rebounds in a 128–121 loss to the Houston Rockets. On 3 February, that season-high would be surpassed with a 38-point, 12-rebound performance in a 112–103 win over the Indiana Pacers. Just two days later, Porziņģis would follow that up with 32 points and 12 rebounds in a 121–107 loss to the Memphis Grizzlies, before leaving the game with a broken nose. On 1 March, Porziņģis matched his than season-high 38 points, along with 14 rebounds and 5 blocks, in a 111–91 victory over the Minnesota Timberwolves. Three days later, he registered 34 points, 12 rebounds and 5 blocks in a 127–123 overtime win over the New Orleans Pelicans, becoming the first player since Shaquille O’Neal in 2000 to have consecutive 30-point, 5-block games. On 31 July, Porzingis recorded a season-high 39 points, as well as 16 rebounds, in a 153–149 overtime loss to the Houston Rockets. This was the Mavericks’ first game in the Orlando bubble, returning from a four-month hiatus due to the COVID-19 pandemic.

In March 2019, it was publicly revealed that a woman accused Porziņģis of raping her at the Sky building in February 2018, hours after he tore his ACL. The woman reportedly told police she waited more than a year to come forward because she had discussed getting a $68,000 payout from Porziņģis to keep quiet. Porziņģis’ attorney, Roland G. Riopelle, publicly denied the claim, and said he had previously referred the case to federal authorities due to the “accuser’s extortionate demands”.