Story highlights Guinan has been marching in the Pride Parade for years

She came out as a transgender woman three years into her firefighting career

(CNN) When Brooke Guinan joined the New York City Fire Department in 2008 she publicly presented herself as a man. She had no idea that on Sunday she'd be one of the NYC Pride Parade's grand marshals while identifying as a transgender woman.

Guinan began identifying as a transgender woman in 2011, three years into her firefighting career at FDNY. She first came out as a gay man at a young age, but began to question her gender identity in college.

Before joining the department, Guinan was unsure what her professional life would look like.

Despite being a third generation firefighter Guinan did not think there was a place for LGBTQ people in the male-dominated fire department.

But Guinan's love of public service ultimately drove her to continue her family legacy in the fire department. There was no LGBTQ training in the beginning.

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