From Clifton, New Jersey, Miazga attended Clifton High School for one year before he left and joined the New York Red Bulls Academy in 2009 at under-14 level. While with the under-16 side, Miazga helped the Red Bulls win the U.S. Soccer Development Academy National Championship in 2012.

Internationally, Miazga has represented the United States at under-14, 15, 18, 20 and 23 levels. He also played one match for the Poland under-18 team in 2012. In May 2013, Miazga played for the United States under-18 side that participated in the 19th Annual International Juniors Tournament in Portugal.

In 2013, Miazga had signed a letter of intent to play for the Michigan Wolverines men’s soccer program. However, after offers from Europe and the New York Red Bulls, Miazga turned down his scholarship offer to play in college and signed with the Red Bulls at the professional level.

On May 30, 2013, it was announced Miazga had signed a homegrown contract with the New York Red Bulls, becoming the eighth player to sign a homegrown contract with the club. Miazga made his first-team debut at age 18 as a substitute for Markus Holgersson in the 76th minute of a 4–1 win at Houston Dynamo on September 8, 2013.

Miazga started playing more frequently during the 2014 season. Also, in August 2014, he signed an endorsement deal with Nike and played in the Major League Soccer Homegrown Game. Miazga made his CONCACAF Champions League debut in a 1–1 draw against the Montreal Impact on October 22.

In August and September 2014, Miazga turned down two separate call-ups to the Poland under-20 team. He cited previous commitments to the United States under-20 team and maintained he had not made a final decision about which nation to represent at senior level. In October 2014, Poland U-20 coach Marcin Dorna stated he had not given up on Miazga and that the Red Bulls player might still play for his team. Miazga played for the U.S. under-20 team at the 2015 CONCACAF U-20 Championship and helped the team qualify for the 2015 FIFA U-20 World Cup in New Zealand. Miazga was a standout performer at the 2015 U-20 World Cup, in which the Americans fell in the quarter finals to eventual champions Serbia on penalties.

During the 2015 season, Miazga established himself as a starter for the club, forming a partnership in the backline with Damien Perrinelle. He was sent off for two bookings in a 2–1 victory against New York City FC in the first ever Hudson River Derby match on May 10. On June 28, he scored his first goal as a professional in a 3–1 derby victory over the same opponents at Yankee Stadium. He celebrated by pretending to swing a baseball bat, making fun of the fact that their rivals play in a baseball stadium. Miazga was sent off for the second time of the season on September 25 in a 5–2 loss to Orlando City SC at the Red Bull Arena.

On November 5, 2015, Miazga was quoted as saying, “I’m totally undecided. I’ll wait to see who calls me first to the main national team and see what happens then.” He received his first United States call-up for the 2018 FIFA World Cup qualifiers matches. He earned his first senior cap against Saint Vincent and the Grenadines on November 13, 2015, coming on as a 63rd-minute substitute in a 6–1 win at Busch Stadium in St. Louis. On July 15, 2017, Miazga scored the Americans’ third goal in a 3–0 win over Nicaragua in the 2017 CONCACAF Gold Cup

On January 30, 2016, Miazga joined English Premier League club Chelsea on a four-and-a-half-year deal for a reported £3.5 million (US$5 million). Three days later, he was given the number 20 shirt for the remainder of the 2015–16 campaign.

He was first included in a Chelsea matchday squad on February 16, remaining an unused substitute in their 2–1 loss at Paris Saint-Germain in the first leg of the round of 16 of the UEFA Champions League. With both John Terry and Gary Cahill injured, Miazga made his Chelsea debut starting in a 4–0 Premier League victory away to Aston Villa on April 2, 2016. After an impressive debut, Miazga started his second match against Swansea City, but was substituted at half-time.

On August 31, 2016, Miazga was loaned to Dutch Eredivisie club Vitesse for the 2016–17 season, after a deal with Espanyol fell through due to paperwork. On September 10, Miazga made his debut for Jong Vitesse in a 2–2 draw against SV Spakenburg in the Tweede Divisie The following day, he was on the substitutes’ bench for the first-team against Ajax, although he was an unused substitution as Vitesse lost 1–0. He made his Eredivisie debut on September 25, coming off the bench in the 64th minute and provided an assist to Lewis Baker in a 2–1 away defeat to Twente. On December 14, Miazga scored his first and only goal for Vitesse in a KNVB Cup tie against Jodan Boys, which finished 4–0.

On July 28, 2017, Miazga’s loan spell at Vitesse was extended for the 2017–18 campaign.

On August 6, 2018, Miazga joined Ligue 1 club Nantes on a season-long loan deal. Miazga made his Ligue 1 debut on August 18, 2018, starting and playing 90 minutes in a 2–0 loss to Dijon FCO.

On January 25, 2019, Miazga joined Championship club Reading until the end of the season after returning from Nantes. After a successful loan spell, Reading decided to loan him again for the 2019–20 season. On October 19, 2019, Miazga scored his first goal for Reading in a 1–0 win over Preston North End.

On October 3, 2020, Miazga joined Belgian side Anderlecht on loan until the end of 2020–21 season.