Brattan, who has spent the last three seasons at Melbourne City, was also on the radar of crosstown adversaries Western Sydney Wanderers.
But a late swoop by Sydney derailed that move to add another layer of animosity between the rivals, already heightened by the Sky Blues scooping up former Wanderers playmaker Alexander Baumjohann.
Sydney have also announced defender Ryan McGowan and midfielder Anthony Caceres on two-year deals over the past week.
But it was the loss of Josh Brillante to Melbourne City which brought them into the market for fellow midfielder Brattan.
It’s understood Brattan - who was initially hoping to move to Asia - has agreed a multi-year deal with the defending champions.
Having been bought by Manchester City in 2016, he was quickly loaned back to their Melbourne affiliate via Bolton Wanderers.
The Brisbane Roar product made 26 appearances last season, scoring four times.
Meanwhile, Reid - who was on Sydney’s books in 2012 - replaces club great Terry McFlynn as football director and will also fill the assistant coaching chasm left by Ufuk Talay’s departure to Wellington Phoenix.
Having played in the UK for Brighton and Bradford, as well as Wollongong, Adelaide United and Melbourne Heart in the A-League, the 39-year-old has been with NSW NPL side Rockdale City Suns since 2013.
During that time he has transitioned from playing into coaching.
The twice-capped Reid has also kept his hand in with the Sky Blues in a community football officer role.
The new job-sharing gig, working alongside head coach Steve Corica whilst helping coordinate player recruitment and retention, is a significant step up.
His predecessor McFlynn has joined Perth Glory to take charge of the W-League team and to head up the club’s Academy.
Sydney FC were contacted for comment.
