The Lancastrian’s last coaching appointment was at Nottingham Forest, but he left the City Ground following Billy Davies’ departure in March 2014 alongside fellow back-room staff members David Kelly and Pete Williams.
Bolton-born Darby, 47, also worked alongside Davies during their time together at Preston North End and Derby County.
Following spells with hometown club Bolton as an under-16s academy coach, he assisted former Trotters’ team mate Jimmy Phillips during his brief stint as caretaker manager following Owen Coyle’s sacking in October 2012.
He returned for a second spell at Forest as first-team coach to Davies in February 2013 and spent 13 months with the Reds.
Darby is well known to Whites’ assistant head coach Rob Kelly from their time together in the East Midlands.
In his playing days, Darby was a midfielder with Bolton, Coventry, West Brom, Preston and Carlisle.
So far this pre-season, Stuart Hayton has come in as the new club secretary at Elland Road, while Martyn Glover has arrived at the club as the head of recruitment.