It isn't known what happened to Marty later in his life. Since he had not committed a crime and in fact tried to prevent a murder, Marty wasn't arrested. And Lt. Muldoon offered to put in a good word for him with Mr. Cienzi, Marty's former boss in the garment district.
"Marty Nero" was not his real name, according to Toobworld Central. Like all gang members, he had taken an alias. But his nickname was based on his real name, the surname being "Martin".
Marty had an older brother named James Martin who walked the straight and narrow growing up. He became a special agent for the government who got involved with secret agent Amos Burke during a very special case in 1965:
A peace group planned on stealing an LSD gas to use on the populace of Washington, DC, in order to cover their plans to steal military secrets. It was Amos Burke's plan to infiltrate the group.
However, it turned out that Special Agent James Martin wasn't the All-American Boy Scout he appeared to be. He tried to arrest Burke after Amos made contact with the real leaders of the peace group. During their struggle, Burke accidentally shot Agent Martin and killed him, only to find out later that Martin was a double agent.
As for Marty's chosen surname of "Nemo", he picked it because Captain Nemo had been his favorite character when he was growing up. And Marty may have imagined himself one day being the captain of the gang.
Marty had no clue that Nemo wasn't fictional, but instead that he not only was a nautical and scientific genius back in the 1850's to rival Dr. Loveless, but that he was still alive (although trapped in suspended animation undersea in the Nautilus.)
- 'Naked City' - "No More Rumbles"
- 'Amos Burke, Secret Agent' - "Peace, It's A Gasser"
- "The Amazing Captain Nemo"