Having come through the academy system of the New York Red Bulls, he made his debut in the 2013 Major League Soccer season and won two MLS Supporters' Shields with the team.

[3] 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.

[20] 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.

[21] 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.

[23] On August 31, 2016, Miazga was loaned to Dutch Eredivisie club Vitesse for the 2016–17 season,[24] after a deal with Espanyol fell through due to paperwork.

[27] 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.

[37] On August 5, 2022, it was announced that Miazga had returned to the United States, signing with MLS side FC Cincinnati on a three-and-a-half-year deal.

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 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.

[47] 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.