Antonini signed with the Mets in 2007 and made his professional debut with their rookie league team, the Kingsport Mets, that year. After a few starts he was promoted to the Brooklyn Cyclones. He split 2008 between the Savannah Sand Gnats and the St. Lucie Mets. He spent most of 2009 with the AA Binghamton Mets, where he was 7–5 with a 5.32 ERA in 25 appearances. In 2010, he was 6–9 with a 4.32 ERA in 23 starts for Binghamton and received a promotion to the AAA Buffalo Bisons where he started six games late in the year and was 2–3 with a 5.11 ERA.
Antonini was traded to the Los Angeles Dodgers on December 27, 2010, for Chin-lung Hu. With the Dodgers organization in 2011, he made 27 starts for the AA Chattanooga Lookouts and was 10–9 with a 4.01 ERA. He played in the mid-season Southern League All-Star Game and was added to the Dodgers' 40-man roster after the season to protect him from the Rule 5 Draft. Antonini was promoted to the AAA Albuquerque Isotopes to start 2012. On April 24, he was called up to the major leagues for the first time when Matt Guerrier was placed on the disabled list; however, Antonini was optioned back to the minors on April 27 without appearing in any games. On May 28, he was again called up to replace the injured Ted Lilly, but returned to AAA one day later, again without appearing in a game. He was designated for assignment by the Dodgers on July 31, and removed from their 40-man roster. Because Antonini has spent time on an active MLB roster without ever appearing in an MLB game, he is an example of a "phantom ballplayer." Antonini cleared waivers and was outrighted to the Isotopes. He pitched in 30 games in Albuquerque, including 13 starts, and finished the season 2–7 with a 5.71 ERA. He was released at the end of March 2013.
During 2015, Antonini was briefly with the Arkansas Travelers, the AA affiliate of the Los Angeles Angels. In four games he had a 1–1 record with 4.95 ERA.
During 2015 and 2016, Antonini was with the Bridgeport Bluefish; during those two seasons he made a combined 28 appearances with an 8–12 record and 3.43 ERA. He signed with the Somerset Patriots in 2017, and re-signed with the Patriots for the 2018 and 2019 seasons.
On July 12, 2019, Antonini’s contract was purchased by the Pericos de Puebla of the Mexican League. He was released on September 2, 2019.
Third stint in Atlantic League
On September 3, 2019, Antonini re-signed with the Somerset Patriots of the Atlantic League of Professional Baseball. He became a free agent following the season.
