Davis attended Chaparral High School in Scottsdale, Arizona, where he won three state titles (2003–05). His father would throw him batting practice. He hit .559 as a sophomore, the school record, ahead of Paul Konerko’s .558 in 1994. He also hit a school-record 23 doubles, breaking Konerko’s record of 18. He followed that up by batting .425 as a junior (when he was also Arizona 4A Pitcher of the Year), and .450 as a senior. As a senior, he had a 92–93 mph fastball, to complement his changeup and slider. He was ranked 12th in the country by Baseball America, and was a high school All-American.

In 2003, he also pitched for the U.S.A. Youth National Team (16-under) in international play. They won the gold medal in the International Baseball Federation XI “AA” World Youth Championships in Taiwan. In 2004, he played on the U.S.A. Junior National Team (18-under). In his two seasons playing for Team U.S.A., he batted .404.

In 2004, he was one of 40 players from across the country chosen to play in the AFLAC All-American High School Baseball Classic. He won the MVP Award for the game, hitting the go-ahead home run for his team. The following year, he was MVP in the all star 2005 High School American Game.

By November 2004, the 17-year-old Davis was already 6 ft 4 in (1.93 m) and 194 pounds. In 2005, despite his having indicated he was going to go to college, he was drafted in the 19th round by the Tampa Bay Devil Rays. He elected not to sign.

Davis chose to attend Arizona State University (ASU). In October 2005, he was ranked the # 2 freshman in the nation by both Baseball America and Collegiate Baseball. He pitched (as the team’s Friday night starter; the role given the “ace” of a college rotation), was the designated hitter, and played first base and corner outfield. In March 2006, he was named a Louisville Slugger National Player of the Week, Pac-10 Conference Player of the Week, and College Baseball Foundation National Honor Roll Player of the Week for a week in which he batted .588 and drove in 13 runs, in four games.

In 2006, with 65 RBIs in 227 at bats he became the first freshman ever to lead the Pac-10 in runs batted in during the regular season, and set the ASU freshman RBI record. Batting clean-up, he hit .329 with 79 hits (third all-time, for an ASU freshman), 23 doubles (tied for the Pac-10 lead, and tying the ASU record for doubles by a freshman), and a .542 slugging percentage in 58 games. His 9 home runs tied him with Bob Horner (1976) for third all-time by a Sun Devil freshman, 2 behind Barry Bonds (11, in 1983). He was also the team’s opening day starter, and pitched a team-high 12 starts. He was named a Collegiate Baseball Louisville Slugger Freshman All-American, Rivals.com First-Team Freshman All-American, Baseball America Second-Team Freshman All-American, Jewish Sports Review First-Team All-American, American Baseball Coaches Association (ABCA) First-Team All-West Region, Pac-10 Freshman of the Year, and a member of the First-Team All-Pac-10.

He hit .353 in his college career, with a .605 slugging percentage (10th-best in ASU history). He totaled 159 runs (8th-best), 244 hits, 33 homers (5th), 69 doubles (2nd, behind Dustin Pedroia), and 202 RBIs (3rd). On the mound, he ended his college career as the Sun Devils closer, and totaled a 7–5 mark with 4 saves and 78 strikeouts in his career. Davis was also a part of two Pac-10 Championship teams, and went to the College World Series in 2007. He was named to the ASU All-Decade team.

He was named a pre-season 2008 All-Pac-10 outfielder by Rivals.com, and a pre-season Third-Team All-American by both the NCBWA and Collegiate Baseball. In consecutive weeks in March 2008 he was named both the Louisville Slugger National Player of the Week and the Pac-10 Conference Player of the Week. In the first week he hit .450 and struck out all four batters he faced. In the second week he batted .529 with a 1.412 slugging percentage, and recorded five outs from the mound, four by strikeout, without giving up a hit. He was the first Sun Devil to win the Pac-10-award in consecutive weeks since Travis Buck in 2004. The College Baseball Foundation named him to its National All-Star Lineup.

In 2009, Davis started the year with the St. Lucie Mets. He began to turn it around, hitting .289 with 7 home runs in 59 games. He was then promoted to the Double-A Binghamton Mets, where he came into his own, hitting .309 with 13 home runs, 41 RBIs, and a .565 slugging percentage in half a season. Mako Oliveras, the B-Mets manager, described him by saying: “Very live bat; the ball jumps off his bat when he makes contact. And as for defense, he’s like a vacuum cleaner.” For 2009, Davis was named the Mets Organizational Player of the Year.

After the season the Mets assigned him to the Surprise Rafters of the Arizona Fall League, where he hit .391. He was named to the Arizona Fall League’s all star Rising Stars Game. In September 2009, he played for the gold-medal-winning U.S.A. World Cup team. In the off-season, Baseball America rated him the Mets’ No. 4 prospect.

Davis played in spring training with the Mets in 2010, and led the team with a .480 average as he hit 3 homers. He also sparkled on defense, prompting José Reyes to observe: “People talk about his hitting, but he is one of the best defensive first basemen you will ever see for a player his age.” Many Mets players were rooting for Davis to break camp with the team, but he was sent down to the minors at the end of spring training. On April 19, however, the Mets purchased Davis’s minor league contract. At that point Davis had played only 65 games in his life above the Single-A level.

Davis was named the first baseman on Baseball America’s 2010 All-Rookie Team. He came in 7th in the voting for 2010 NL Rookie of the Year, receiving two third-place votes.

In 2011, Davis also appeared on an episode of the fourth season of MTV’s game show Silent Library.

For the week ending June 24, 2013, Davis was the Pacific Coast League’s Player of the Week. In 2013, in 10 games with the AAA Las Vegas 51s, he batted .364 with a .500 on-base percentage and a .636 slugging percentage.

Davis struggled early in 2013 and became a subject of criticism for many Mets fans and sports analysts. In June, Davis was sent down to Triple-A Las Vegas for 10 games, in which he batted .364. He was recalled in early July. He tied a team record set by John Olerud in 1998, by reaching base at least twice in 12 straight starts from July 29 – August 13. He recovered to hit .286 in the second half, with a .449 on-base percentage. For the season, he batted .205 with 9 home runs and 33 RBIs for the Mets, in 103 games.

On April 18, 2014, the Mets traded Davis to the Pittsburgh Pirates in exchange for Zack Thornton and a player to be named later, Blake Taylor. Davis hit another grand slam against the Reds on April 21, becoming the third player in MLB history to switch teams midseason and hit a grand slam against the same opponent (following Ray Boone in 1953, and Mike Piazza in 1998), and the first player to hit grand slams for different teams in the same April. Overall, in Pittsburgh he hit .235 with 10 home runs and 46 RBIs in 336 at-bats, splitting time with Gaby Sanchez at first.

Davis was designated for assignment by the Pirates on November 20, 2014.

On November 23, 2014, the Pirates traded Davis to the Oakland Athletics in exchange for bonus slots for use in signing international free agents.

On April 21, 2015, Davis pitched a perfect eighth inning in the losing side of a blowout against the Los Angeles Angels of Anaheim, retiring all 3 batters he faced, all on ground balls, using only 9 pitches. Davis in high school was 23–0 with a 1.85 ERA and was the Arizona High School Pitcher of the Year, and in college he had a 2.25 ERA as a pitcher. Against the Angels he threw his fastball as high as 88 miles per hour, threw a slider in the high 70s, and threw a cutter. It was the first time the A’s had a position player pitch since Frank Menechino in 2000.

On August 21, 2015 it was announced Davis would undergo surgery to repair a torn hip labrum, ending his season.

On February 15, 2016, Davis signed a minor league contract with the Texas Rangers. Hampered by injuries throughout spring training, Davis was assigned to the Triple-A Round Rock Express to begin the season. On June 12, Davis was released from his contract in order to pursue a major league opportunity elsewhere. In 39 games with the Express, Davis hit .268/.350/.437 with 4 HR, 25 RBI, and 37 strikeouts.

On June 12, 2016, Davis agreed to a major league contract with the New York Yankees. The Yankees depth at first base was very thin, with four first baseman on the disabled list, and middle infielder Rob Refsnyder manning the position at the major league level at the time of his signing. He was designated for assignment by the Yankees on June 25. Two days later, he was outrighted from the 40-man roster to the Scranton/Wilkes-Barre RailRiders of the Triple-A International League. On August 10, Davis was released by the organization.

Davis signed a minor league contract with the Los Angeles Dodgers on January 27, 2017. He began the 2017 season playing first base for the Oklahoma City Dodgers of the AAA Pacific Coast League. In 35 games, he hit .212/.258/.412 for Oklahoma City. After the season, Jeff Passan of Yahoo Sports ranked him the 13th-best free agent first baseman.

Davis played for Israel at the 2017 World Baseball Classic in March 2017. He batted .471/.571/.706 with 2 doubles and a triple, scoring 5 runs and driving in 3 RBIs in 17 at bats.

In midseason, Davis switched to pitching and made his debut on the mound for the Arizona League Dodgers, pitching one scoreless inning, striking out the side as his fastball was in the 88-92 mph range. He pitched in six games in the Arizona League at the end of the season, allowing no runs and only three hits in 5 ⁄3 innings, with six strikeouts. LA Dodgers catcher Kyle Farmer said: “He has a live fastball, really good changeup, and he’s working on his slider right now.” On November 6, he elected to be a free agent. He retired in November 2018.