Ridge announced his retirement on Tuesday "to give personal and family matters a higher priority." He said he would remain in office until Feb. 1 or until the Senate confirms his successor.
President Bush praised Kerik as "one of the most accomplished and effective leaders of law enforcement in America" at a White House ceremony Friday morning with the former police commissioner at his side.
"In every position he has demonstrated a deep commitment to justice, a heart for the innocent and a record of great success," the president said, according to the Associated Press. "I'm grateful he's agreed to bring his lifetime of security experience and skill to one of the most important positions in the federal government."
Kerik said his experiences as police commissioner during 9/11 would stay with him if he were confirmed to the homeland security post.
"Both the memory of those courageous souls and the horrors I saw inflicted upon our proud nation will serve as permanent reminders of the awesome responsibility you place in my charge," he said.
Kerik's background is far different than Ridge, who is a Harvard-educated, former congressman and governor of Pennsylvania.
Kerik grew up in Paterson, N.J., spending his childhood without a mother and with relatives and friends until his father was awarded custody, according to a BBC profile. His mother, he later discovered, had been an alcoholic prostitute who was murdered in Ohio.
Kerik dropped out of high school to join the army, where he became a military policeman stationed in South Korea, and later a security expert in the Middle East. But his desire to be a policeman took him to the New York Police Department in 1986. He became a star undercover narcotics detective who helped bring down members of Colombia's Cali cartel, according to the BBC.
In the early 1990s, he began a long professional association with former New York Mayor Rudolph Giuliani, serving as his campaign bodyguard. He then joined the Department of Correction as the director of investigations.
In 2000, Giuliani chose him to take over as the city's police commissioner. During his 15-month tenure, he was praised for his role in reducing crime rates in the city.
In the wake of the Sept. 11 attacks, he caught the eye of the White House and was given a role as special policy adviser to the Iraqi interim government to help set up a fledgling police force.
Kerik, 49, has a grown son, and two young daughters with his second wife Hala.
The nomination came as the president's second term Cabinet continued to change. Health and Human Services Secretary Tommy Thompson reportedly resigned just hours after Kerik's announcement. Thompson becomes the eighth Cabinet official to step down since the president's re-election.
Dr. Mark McClellan, who is currently in charge of the office within HHS that administers the federal Medicare and Medicaid programs, is considered the likely candidate to replace Thompson, according to The New York Times.