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.
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.
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.
After strong performances against New York City FC's David Villa and Toronto FC's Sebastian Giovinco, Miazga became regarded as one of the best defensive players in the league and earned MLS team of the week appearances four times throughout the season. His strong defensive play throughout the 2015 season helped New York win their second Supporters' Shield in three years by having one of the strongest defenses in the league. At the end of the season, the Red Bulls offered Miazga a designated player deal which would have made him one of the highest players on the team. However, Miazga turned down the offer, stating his desire to play in Europe.
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.
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 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.
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
During his time at the Red Bulls, Miazga lived with his parents Adam and Elżbieta in Clifton. His parents are Polish-Americans. Miazga's idols are Rafael Márquez, Carles Puyol and Gerard Piqué.As of September 20, 2017
As of July 15, 2017
Scores and results list the United States's goal tally first.
New York Red Bulls
Supporters' Shield (2): 2013, 2015
Eastern Conference (regular season) (2): 2013, 2015
KNVB Cup (1): 2016–17
CONCACAF Gold Cup (1): 2017