John Robertson came back to haunt his former club by scoring the Scottish Cup winner at Victoria Park.
Robertson rose to meet a Mark Roberts cross after 15 minutes to send a free header into the net.
County had strong claims for a penalty turned down after 38 minutes when Michael Gardyne appeared to be upended by Jags goalkeeper Jonathan Tuffey.
Jags striker Roberts saw his late goal-bound effort cleared off the line via the upright by Kevin McKinlay.
Ross County: Samson, Smith, McKinlay, Dowie, Webb, McCulloch (Gunn 66), Gardyne, Adams, Higgins (O'Carrol 58), Cowie, Moore. Subs Not Used: Malin, Scott, Anderson.
Partick Thistle: Tuffey, Smith, John Robertson, Brady, Boyd, McChrystale, Billy Gibson, Ferguson, Young, Roberts (McCulloch 89), McConalogue (Graham Gibson 80). Subs Not Used: Arthur, Kane, Hodge.
Goals: John Robertson 15.
Ref: C MacKay