Harmison finished with two for 47 off 10 overs as the hosts made 307 for five.
Harmison made the breakthrough, taking Mark Chilton on 19, but centuries by Mal Loye (127) and Brad Hodge (118) allowed the hosts to set a daunting total.
Despite an opening stand of 64 by Jonathan Lewis and Jimmy Maher, the visitors' task was rendered improbable after they lost four wickets for just three runs with Glen Chapple and Simon Marshall taking three wickets apiece.
Lewis (37), Maher (34) and Phil Mustard (33) top-scored, with Harmison making 17.
David Sales' century was in vain as Yorkshire held off the challenge of Northamptonshire to win by two runs in a high-scoring game.
Darren Lehmann (118 not out) scored his second ton in a week and with Craig White (112) helped set an improbable total of 341 for three.
Northamptonshire were reduced to 90 for three but Sales shared a stand of 215 with Chris Rogers as the pair made 161 and 85 respectively.
However, they lost three wickets, including that of Sales, for just eight runs and Yorkshire held on to win.
Naved-ul-Hasan took five for 30 as Sussex beat Glamorgan by 97 runs at Sophia Gardens.
The Pakistan fast bowler wreaked havoc on the visitors who had set a decent total of 257 for eight mainly thanks to a stand of 126 between Michael Yardy (88) and Carl Hopkinson (61).
Sussex were not allowed to settle and only a stand of 55 between Richard Grant (36) and David Harrison (30) prevented a total humiliation after they were reduced to 78 for seven.