After Cardin was
admitted to the Maryland Bar in 2001, he clerked for Baltimore City Circuit Court Judge
William D. Quarles Jr. until 2002. Afterwards, he started his own law firm, Jon S. Cardin, P.A. which he attributed to him leaving early to recognize
Shabbat and to spend time with his daughter and pregnant wife. He won the Democratic primary election on June 26, 2018, placing third with 23.6 percent of the vote. He won the general election on November 6, defeating Republican challenger Jonathan Porter with 29.3 percent of the vote. Cardin was sworn in for his second stint in the legislature on January 9, 2019, during which he has served on the Judiciary Committee. In January 2020, the Baltimore County Democratic Central Committee voted to nominate
Shelly Hettleman to fill the vacancy over Cardin. In August 2024, after
Johnny Olszewski won the Democratic nomination in the
2024 U.S. House of Representatives election in
Maryland's 2nd congressional district, Cardin expressed interest in serving the remainder of Olszewski's term as
Baltimore County Executive. Cardin applied to succeed Olszewski, but withdrew from the race in late November 2024.
Baltimore Police marriage proposal incident In August 2009, Cardin was criticized for using
Baltimore Police Department resources to perform a marriage proposal prank. Cardin was using a friend's boat when police boarded the boat as a Foxtrot helicopter hovered above the boat. According to the
Baltimore Sun, Cardin's girlfriend thought she was going to be arrested when Cardin proposed to her. After the issue became public, Delegate Cardin promised to reimburse the City of Baltimore for the costs incurred. Cardin paid $300 to the Baltimore Police Department to cover the costs and donated $1,000 to the city's mounted unit. The Baltimore Police Department launched an internal affairs investigation into the marriage proposal incident. In November 2009, a police sergeant was charged with misconduct in connection with the prank.
2014 Maryland Attorney General campaign In November 2012, Cardin formed an
exploratory committee into a potential run for
attorney general of Maryland in
2014. He officially announced his candidacy on July 22, 2013, becoming the first candidate to enter the race. Cardin ran on a platform that involved focusing on
cyber fraud,
identity theft, and public safety. for misusing
Baltimore Police Department resources in a stunt wedding proposal, missing 75 percent of committee votes during the 2014 legislative session, and touting an endorsement from Ski Money, a Baltimore-based rapper who had been indicted on human trafficking charges in Baltimore County. Following the Democratic primary election, he called these criticisms "the most negative smear campaign in a Democratic Party primary in Maryland's modern history". In June 2014, Cardin filed a report with the Maryland State Board of Elections against state senator
Lisa Gladden and state delegate
Maggie McIntosh, both supporters of
Brian Frosh, accusing the lawmakers of "verbally harassing" one of his campaign volunteers. Both legislators denied Cardin's accusations, with McIntosh saying that she had not confronted any Cardin supports while canvassing and Gladden saying that she was never at the polling place described in the report. Polling ahead of the primary election showed Cardin leading other Democratic challengers Frosh and
Aisha Braveboy, but with a majority of voters saying that they were undecided. He was defeated in the Democratic primary by state senator
Brian Frosh on June 24, 2014, and conceded to Frosh the day after the election. Following his defeat, Cardin worked as a lobbyist. ==Political positions==