2015 was a breakout year for McNulty when he won the Valley of the Sun Stage Race TT, averaging 30 mph on a standard road bike.

In 2016, McNulty had even more success, winning the Tour de l'Abitibi and Trofeo Karlsberg, stage races, and the junior national time trial championships for the second year in a row.

In 2019, McNulty took his first win as a professional at the newly revived Giro di Sicilia in the penultimate stage to Ragusa, thus taking the lead in the general classification.

He entered the 2022 Tour de France as one of the primary Lieutenants for Pogačar, along with Majka, Soler and Bennett; however all of them were out of the race by the critical final two high mountain stages.

On stage 17, which included Col de Val Louron-Azet to Peyragudes, McNulty drove a pace that broke the entire elite GC group with the exception of his team leader Pogačar, and Jonas Vingegaard.

[10] In May 2023, he took his first win in a Grand Tour, outsprinting Ben Healy and Marco Frigo from a three-man breakaway on stage 15 of the Giro d'Italia.

[12] McNulty had a strong start to 2024, winning the Volta a la Comunitat Valenciana in February, after taking the race lead with his victory on stage four.

[14] On the final weekend of March, McNulty won the GP Miguel Induráin, beating out Maxim Van Gils to become the first American to win the event.