With Northants 9-0 at the start of the third day at Lord’s, the visitors needed 294 to win the Division Two encounter but were bowled out for 142 to lose by 160 runs.
James Harris (4-39) completed a 9-48 match haul following his stunning 5-9 in the first innings, while Ireland veteran Tim Murtagh (4-36) took eight wickets in the match.
Northants will reflect that the game was lost in the first innings. Harris had compiled an unbeaten 46 to help the hosts scramble up to 214 having been inserted on the first day. But in conditions that suited Middlesex’s seam attack, Northants gave up a mammoth lead as they succumbed for a paltry 71 all out.
Middlesex’s moderate second-innings 159 put them miles ahead and Northants’ best score in the final dig came from Rob Newton, who made 44.
It was a strong start to the season for Middlesex, who were relegated from Division One in 2017, just one year after sealing the overall County Championship title.