The former homicide detective, who was booted off the Democratic line due to a mistake on his voter forms, said he was "honored" for the endorsement from Ulrich, one of the few Republicans in the City Council who also briefly mulled a run for mayor.
"Two guys from Ozone Park," Dietl said, before a press conference on the steps of City Hall.
"To me, this is one of the biggest endorsements I can get."
Dietl aspires to join a crowded field of Republican challengers that includes former New York Jet Michael Faulkner and real estate executive Paul Massey.
As a registered Democrat, he would need to obtain a Wilson Pakula, which authorizes him to run on the Republican line.
Despite the challenges, Ulrich believes he has the best chance to defeat Mayor Bill de Blasio in his re-election bid.
"I absolutely believe Bill de Blasio can be beat this year, and I think the only person who can beat him that's currently in the race for mayor is Bo Dietl, and that's why I'm supporting him," he said.
Voters "want a mayor who wants to be a mayor for all New Yorkers," Ulrich said, taking shots at de Blasio's fundraising in cities across the country.
"We need a mayor who actually wants to be mayor, and not raise his national profile or national political image," he said.
Dietl also took shots at de Blasio, who he's called "Big Bird" in the past (although he said it only once at Monday's press conference.)
He also touched on a wide range of topics and issues, jumping from a discussion on property taxes to the opioid epidemic in the same sentence.
He suggested hiring retired police officers to "straighten out" the Administration for Children's Services. Rikers Island should be fixed from within, instead of closing it, he said.
He accused the mayor of "playing" with crime statistics, calling de Blasio a "lazy mayor" who arrives to work in the afternoon.
Although he has supported fellow Queens native President Donald Trump, Dietl said he hasn't spoken to him since he was elected.
And voters shouldn't align him with the president, although he did say he would try to work with him if he's elected.
"I'm not another Donald Trump, I'm Bo Dietl, and I spilled my blood for this city," he said.
He said he ultimately cares about the city — and will do anything to win the trust of New Yorkers.
"I'm going to the Dominican Republic, I love Dominican, I love Puerto Rican, I love black," he said.
"I'll go and do anything, I'll come anywhere, for anybody, to let you see, I want to be your mayor."