The St. Louis Cardinals selected Maness in the 11th round of the 2011 MLB draft, and Maness signed with the Cardinals. Maness started his professional career with the Batavia Muckdogs of the Class A-Short SeasonNew York–Penn League. After appearing in the All-Star Game, the Cardinals promoted him to the Palm Beach Cardinals of the Class A-AdvancedFlorida State League. He finished the season with the Quad Cities River Bandits of the Class AMidwest League. Maness is noted for his control: in 169 2/3 innings pitched in 2012, he allowed only ten walks. Maness was named to the Texas League postseason All-Star team in 2012. On November 30, 2012 Maness was named the Cardinals' Minor League Pitcher of the Year. His efforts on the mound helped Double A Springfield as they were named Minor League Team of the Year by Baseball America. On April 29, 2013, Maness was called up to the major leagues due to the Cardinals' struggling bullpen. Maness made his MLB debut on May 3. He pitched one inning in his debut, against the Milwaukee Brewers at Miller Park in the eighth inning, giving up no hits, and getting three consecutive groundouts for his debut, following the seventh inning debut of Carlos Martinez who was brought up earlier that day. On May 15, 2013 he became the first Cardinal rookie pitcher since 1900 with three relief wins in his first five major league appearances. In 66 games with St. Louis, Maness went 5–2 with 15 holds, 1 save and a 2.32 ERA, striking out 35 in 62 innings. He led the team in ERA. Maness pitched five effective innings in the Cardinals postseason run, posting an earned run average of 1.80. The one earned run he surrendered was a three-run home run to Jonny Gomes in Game 4 of the 2013 World Series. The Cardinals would not lead in a World Series game after this home run, falling in six games. An arbitration-eligible player prior to the 2016 season, Maness and the Cardinals agreed to a one-year, $1.4 million contract on January 15, 2016. After yielding a 6.39 ERA for the season, and his velocity on all four pitches down 2-3 mph, he landed on the DL for the first time in his career on May 14, after an exam revealed an inflammation on his right elbow. He was having pitching difficulties, getting hit hard in his previous inning work against the Anaheim Angels on May 12. For the season, he has a 6.39 ERA and 1.97 WHIP in 12 innings. He will eventually be able to join a Minor League affiliate on a rehab assignment, instead of as a member of that club's roster. That rehab assignment can last up to 30 days. On August 16, Maness was placed on the disabled list with an elbow strain. It was later revealed that Maness would need to undergo Tommy John surgery, effectively ending his 2016 season and likely for most of the 2017 season. After a medical review of his injury, Maness had surgery on August 18, to fix his Ulnar collateral ligament of elbow joint instead of Tommy John surgery ligament replacement, drastically reducing his recovery time from 12–15 months to 6–8 months, which would make him available for Spring Training in 2017. The Cardinals non-tendered him on December 2, 2016, making him a free agent.
On February 13, 2017, the Royals announced that Maness signed a minor league contract, with a $1.25 million incentive for reaching the major leagues. The Royals promoted Maness to the major leagues on May 10. He resigned a minor league deal on November 2, 2017. Maness was released from the organization on May 10, 2018.
On May 1, 2019, Maness's contract was purchased by the Texas Rangers and he was assigned to the Triple-ANashville Sounds. He produced a 8–4 record with a 5.38 ERA over 117 innings for Nashville and was named a 2019 Pacific Coast League All-Star.
