He previously appeared for Sporting Kansas City (SKC), LA Galaxy, Columbus Crew SC, and D.C. United and spent time on loan with Orlando City, OKC Energy, San Antonio Scorpions, Swope Park Rangers, LA Galaxy II, Hartford Athletic, San Diego Loyal, and Loudoun United.

Kempin spent four spells on loan while with SKC, winning a USL Pro championship with Orlando City and also spending time with OKC Energy, San Antonio Scorpions, and Swope Park Rangers.

After being released by Sporting KC, he spent a year in the LA Galaxy organization before joining Columbus Crew SC ahead of the 2018 season.

Born in Leawood, Kansas, Kempin played club soccer for the KC Legends, Blue Valley Stars, and KCFC Alliance youth teams.

[3] He attended Blue Valley North High School in Overland Park, winning a state title and allowing just two goals as a freshman.

He was selected to take part in the United States U17 residency program at the IMG Soccer Academy, where he spent his sophomore year.

[8][10] On March 14, 2013, Kempin was loaned to Orlando City, Sporting KC's USL Pro affiliate, in a bid to find regular playing time.

[14] Kempin returned to Sporting Kansas City with six games remaining in the regular season, but was unable to beat out Jimmy Nielsen or Kronberg to make a matchday squad.

The Energy, playing their debut season in the USL Pro, were coached by Kempin's former teammate in Kansas City, Jimmy Nielsen.

[16] Although he was initially recalled by SKC before playing a game, Kempin returned to make his Energy debut on April 7, starting in a 4–2 defeat against LA Galaxy II.

[22] Kempin went on to appear in SKC's next four games, including a match against Real Estelí in CONCACAF Champions League play on August 20.

[8] On March 20, 2015, Kempin was again sent on loan, this time joining North American Soccer League club San Antonio Scorpions for the duration of the 2015 season.

That revelation, coupled with the selection of Alec Kann in the 2015 MLS Re-Entry Draft, meant that Kempin was again the club's number three goalkeeper.

[31] Kempin started the inaugural game in Swope Park history, making two saves in a 2–1 victory against Portland Timbers 2 on March 26.

[32] After mostly staying with the Rangers for the first half of the season, he spent the early part of August moving between Sporting KC and Swope Park, playing in the league while on loan and appearing on the bench for SKC.

[33] He played twice in the 2016–17 CONCACAF Champions League, with a start against Vancouver Whitecaps FC on August 23 eventually marking his 12th and final appearance with Sporting KC.

[34] Kempin had his contract option declined by the club on November 23, ending his ten-year association with the Sporting Kansas City organization.

[38] However, with Rowe injured and Diop on international duty, Kempin made his MLS debut for the Galaxy on June 3, tallying six saves in a scoreless draw with D.C.

[45] After not making a single appearance through the first two months of the 2019 season, Kempin was sent on loan to USL Championship club Hartford Athletic on April 11.

[50] The Crew acquired two goalkeepers ahead of the 2020 season, and in a bid for playing time Kempin was subsequently loaned out, joining USL Championship expansion side San Diego Loyal on March 6.

[57] Eloy Room and Andrew Tarbell shared the starting job during the club's playoff run, and Kempin also missed the Eastern Conference final after reportedly testing positive for COVID-19.

[66] Before signing his first professional contract, Kempin had participated in training camps with the United States under-15 and under-18 national teams, as well as a stint in the under-17 residency program at the IMG Soccer Academy.

[73] In January 2015, Kempin earned his first call-up to the United States senior national team, with head coach Jürgen Klinsmann including the then-21-year-old as one of four goalkeepers in the squad for a pair of friendlies.