Costello averaged 25.1 points, 19.1 rebounds, 4.0 assists and 4.0 blocks per game at Bay City Western for Coach Chris Watz. Costello led the Warriors to the Class 4A regional semifinals, an 18-6 record, and a second-straight district championship. He finished his career as the school's all-time leader in points, rebounds and blocked shots. He was one of the best recruits in the country. He ranked among the top-100 players in the nation by the following:
As a junior, he averaged 19.8 points, 13.0 rebounds and 3.8 blocks as a junior, shooting 53.4 percent from the field and 72 percent from the foul line. Costello posted a school-record 19 double-doubles. He helped his team improve from 10-11 in 2010 to 24-3 in 2011, leading Bay City Western to Class A state semifinals for the first time in school history. His stats earned him MVP of Saginaw Valley Association. Costello averaged 16.7 points 11.8 rebounds, 3 blocks and 3 assists as a sophomore and set a school record with 14 blocks in a single game. In 2010 Bay City Times named him to the Dream Team and first-team All-Saginaw Valley League selection. He was a National Honor Society member.
College career
Freshman year
As a freshman, Costello appeared in 30 games, averaging 6.1 minutes,1.5 points and 1.3 rebounds, shooting.469 from the field. His.824 free-throw percentage was tops on the team. He ranked fourth on the team with 13 blocks, averaging a block every 14.2 minutes. Playing time increased when he averaged 8.0 minutes in the last 13 games after playing more than eight minutes just once in the first 23 games. Costello turned in one of his best games in a 75-52 win over No. 4 Michigan, scoring a season-high eight points and grabbing a Big Ten season-high six rebounds, hitting all three field goal attempts and both free throw attempts. He scored six points and grabbed a season-high seven rebounds against Tuskegee, blocked a season-high three shots against No. 1 Indiana, and blocked two shots each against Texas Southern, Nebraska and Valparaiso in the second round of the NCAA Tournament. Costello missed the first two games of the season and both exhibition games with a bruised back.
Sophomore year
Costello appeared in 34 games, starting 20, averaging 4.0 points and 3.3 rebounds in 14.7 minutes per contest, He led the team with 43 blocks, ranking 10th in the Big Ten. His 43 blocks were the seventh-best single-season total in school history, and the most ever by a Spartan sophomore. He blocked 24 shots in Big Ten contests, ranking eighth in the conference. Costello ranked second on the team in field-goal percentage In December he missed 4 games due to mononucleosis. He posted his first career double-double in an overtime win at Iowa, scoring a season-high 11 points and grabbing a career-high 12 rebounds; nine of his 11 points came in the last five minutes of regulation and overtime. Costello scored 10 points against Penn State, adding six rebounds and a season-high tying three assists as well as dishing out three assists against Columbia Costello scored six or more points in 10 contests and grabbed five or more rebounds in eight games • Tallied nine points and eight rebounds against Michigan • Also scored nine points against Northwestern • Tallied seven points and eight rebounds against Minnesota He made improvements on playing time and started 13 of 18 Big Ten contests, averaging 4.9 points and 3.6 rebounds in conference games Costello made history when he blocked a career-best six shots at Illinois, tying the second-best single-game effort in MSU history. He blocked two or more shots in 10 contests. At the end of the year, he received Academic All-Big Ten honors.
Junior year
Costello appeared in 39 games, starting six, averaging 7.0 points and 5.0 rebounds in 20.4 minutes. He shot.579 from the floor, ranking second among teammates with more than 100 field-goal attempts. Costello ranked 13th in the Big Ten in blocked shots overall and seventh in Big Ten games. His 48 blocks rank as the seventh-best single-season total in MSU history. Costello managed to score in double figures in 11 games and grabbed double-figure rebounds in two contests. He posted two double-doubles vs. Loyola and Texas Southern. Costello was a team leader in many categories and led MSU in scoring in two games and in rebounding in eight contests. He scored a career-best 17 points against Texas Southern and scored 15 points against Arkansas-Pine Bluff. His season-high of 11 rebounds came against Loyola. He blocked two or more shots in 13 games, including a season-high four against Michigan and Purdue. Costello grabbed seven or more rebounds in 14 contests, including five games with nine or more boards. He started the first six games of the season, then came off the bench for the final 33 contests. At the end of the year, he received Academic All-Big Ten honors. With Costello commanding the post, the Spartans were able to reach the 2015 Final Four.
Senior year
During Costello's senior year, he made many improvements to his game. Because of this, he received many end of the year awards. He is currently Michigan State's all-time leader in blocks. Costello was honored with a selection to the Reese's College All-Star game in Houston, Texas.
After going undrafted in the 2016 NBA draft, Costello joined the Atlanta Hawks for the 2016 NBA Summer League. On July 26, 2016, he signed with the Hawks, but was later waived on October 17 after appearing in two preseason games. He signed with the Memphis Grizzlies on October 20, but was waived again on October 24. Five days later, he was acquired by the Iowa Energy of the NBA Development League as an affiliate player of the Grizzlies.
On July 24, 2017, Costello signed a two-way contract with the San Antonio Spurs. Under a two-way contract, he divided time between the Spurs and San Antonio's G-League affiliate, the Austin Spurs. He made his NBA debut on November 12 during a blowout 133–94 win over the Chicago Bulls.
Costello averaged 11.4 points and 7.8 rebounds per game for Gran Canaria during the 2019-20 season. He inked a two-year contract extension on June 25, 2020.
