G-Man is a slang reference to FBI agents. It stands for "Government Man" and is attributed by FBI myth to "Machine Gun" Kelly who surrendered at one point in 1933 shouting "Don't shoot, G-Men!"
Allan Pinkerton
Allan Pinkerton, (born August 25, 1819, Glasgow, Scotland—died July 1, 1884, Chicago, Illinois, U.S.) was a Scottish-born detective and founder of a famous American private detective agency, the Pinkerton National Detective Agency, still in existence today. He also served as head of the Union Intelligence Service during the Civil War. (Intelligence Service eventually developed into what is known today as the Secret Service.) He died in Chicago in 1884.
From 1929 to 1931 the law enforcement team led by Eliot Ness was known as the Untouchables, a nickname given to describe their incorruptiblity while they aggressively enforced Prohbition laws against Al Capone and his gangsters.
Francesco Vincent Serpico
Frank Serpico is a retired NYPD officer famous for bringing attention to police corruption in the 1960s and 1970s. His actions compelled the mayor at the time, John Lindsay to appoint the Knapp Commission to investigate the police department.
Joseph Dominick Pistone
Known under his undercover alias as Donnie Brasco, Joseph Pistone is a former FBI agent who spent six years infiltrating into the Bonanno crime family. He was a pioneer for long-term, deep undercover work which up to that point had been avoided due to J. Edgar Hoovers fear of corruption to the agents involved.
Unfinished Business by 
Lloyd George Sealy
Lloyd George Sealy commanded the 28th precinct in the 1960s, being only the second black to command a precinct in NYC at the time. In 1966, he became the first Black Assistant Chief Inspector and the first black commander of the Brooklyn North Patrol Service Area, which encompassed 11 Brooklyn precincts. He was the most prominent and visible black police officer in New York at the time. In 1969 Sealy resigned from the police department and joined John Jay College as an Associate Professor of Law and Police Science.
John Edgar Hoover
J. Edgar Hoover, in full John Edgar Hoover, (born January 1, 1895, Washington, D.C., U.S.—died May 2, 1972, Washington, D.C.) was a U.S. public official who, as director of the Federal Bureau of Investigation (FBI) from 1924 until his death in 1972, built that agency into a highly effective, if occasionally controversial, arm of federal law enforcement.
Eliot Ness
Eliot Ness, (born April 19, 1903, Chicago—died May 7, 1957) was an American crime fighter, head of a nine-man team of law officers called the “Untouchables,” who opposed Al Capone’s underworld network in Chicago. He died of a heart attack at the age of 54.
Melvin Horace Purvis, Jr.
Melvin Purvis was an FBI official who led the manhunts of Baby Face Nelson, Pretty Boy Floyd and John Dillinger. J. Edgar Hoover was jealous of the limelight that Purvis attracted which overshadowed him and the rest of the FBI. Purvis died of a gunshot wound to the head in 1960. The FBI ruled it a suicide.
Francis Augustus Hamer
Frank Hamer was the Texas Ranger who hunted down and killed Bonnie and Clyde in 1934. He was considered to be a legendary Texas Ranger and is in the Texas Ranger Hall of Fame. He died in 1955 at the age of 71.
Izzy (Isidor/Isadore) Einstein and Moe Smith (Izzy on the left, Moe on the right)
Izzy Einstein and Moe Smith were agents of the US Prohibition Unit who became famous for their use of disguises to successfully shut down speakeasies. They made close to 5000 arrests and got convictions in 95 percent of the cases. They were eventually laid off by Prohibition Bureau heads who had become jealous of their success and popularity. The two men eventually moved on and worked as insurance salesmen. Izzy died in 1938. Moe died in 1960.
Thomas Edmund Dewey
Dewey was eventually governor of New York and Republican candidate for President in 1944 and 1948 but lost to FDR and Harry Truman respectively. Before he became governor he achieved fame as a federal prosecutor investigating corruption. He made enemies of Dutch Schultz who decided to kill Dewey but was himself killed by the mafia who feared a crackdown if a popular prosecutor like Dewey were to be assassinated. Dewey eventually managed to get Lucky Luciano convicted for running a prostitution racket and got him a 30 year prison term. Dewey was publically popular because he was considered to be honest. He died of a heart attack in 1971.
Albert A. Seedman
A NYPD detective, he was the only Jewish officer to ever hold the position of chief of detectives. Seedman was a prominent face in NYC law enforcement in the turbulent 1960s and 1970s. He died in Florida in 2013.
Detective Thomas Byrnes
Photo from New York City Police, Images of America series, by Joshua Fuff p. 29
Available in Library General Collection, Call Number HV8148 .N5 R84 2011
The father of modern detective work, Thomas Byrnes is also behind the origin of the term "giving someone the third degree", which developed to describe the rough type of interrogations that he preformed.
Detective George Samuel Dougherty
George Samuel Dougherty was the head of the NYPD Detectives Bureau and one of the leading detectives in America in the late 19th and early 20th century.
Lt. Joseph Petrosino (Link)
Guiseppe Joseph Petrosino was the first Italian-American to lead the Homicide division of the NYPD. He was eventually put in charge of the "Italian Squad", a group of Italian-American detectives organized specifically to deal with the Mafia. He was assasinated by the Italian Mafia while visiting Palermo in 1909.
Lt. Samuel J. Battle 1941 (Link)
Samuel Jesse Battle was the first black police officer in the city of Brooklyn, later New York City. In 2009, the 135th and Lenox Avenue in New York City was named after him.
Police Officer Milton Wolf
Photo from New York City Police, Images of America series, by Joshua Fuff p. 78
Available in Library General Collection, Call Number HV8148 .N5 R84 2011
One of the first Jewish policemen to walk a beat in Sheapshead Bay Brooklyn and a member of Shomrim, the fraternal organization for Jewish police officers. His unpublished biography is owned by the New York City Police Museum.
Theodore Roosevelt
President, author, soldier, explorer and historian are just a few of the many roles that Teddy Roosevelt had in his life. In law enforcement, he was president of the board of NYC Police Commissioners from 1895-1897, a period in which he reformed the New York police force. Roosevelt implemented regular firearm inspections, annual physical exams, and recruited on merit rather than political affiliation. He faced down Tammany Hall which moved against him by legislating the Police Commission Roosevelt was on out of existence. When Roosevelt became governor of New York, he established a single Police Commissioner rather than a board. Roosevelt died in 1919.