The midfielder joins from National League South outfit Welling United, who he helped to a third place finish in 2018/19, scoring 15 goals and creating a further 12.
The 26-year-old spent part of his youth career with Fulham and Crystal Palace and cited the ambition of the club as one of the reason's for making the move North.
“When I met with Simon Weaver he told me the club's ambitions and I wanted be part of the journey.
“I've seen the way Harrogate play and I'm looking forward to adding to the squad, helping the boys out and hopefully having a good season here.”
Kiernan made his professional debut for AFC Wimbledon in 2011 at age 18, before making the move to Bromley and hitting double figures for assists in his first season at the club.
Town's newest arrival has also represented Ebbsfleet, Lingfield and Hayes and Yeading before signing for Hampton and Richmond who he helped gain promotion to The National League South, being named Player of the Year along the way, before impressing at Welling.
Here's what you can expect from our new signing...
We would like welcome Brendan to the club and look forward to seeing him in Town colours for the first time in pre-season.
The transfer is subject to FA and National League approval on 1st July 2019.