It leaves Andy Ewen's Stags top of the SCFL division one with four wins from four - and Selsey with work to do to improve on their early-season mid-table spot.
The Stags started well and took a deserved lead in the 11th minute as Lewis Hamilton got free on the left and his shot come back off the post and fell to Marcus Bedford who tapped home.
Selsey were back in the game within a minute - the Stags lost possession 25 yards from goal and the loose ball fell to Jack North who fired home with a well struck shot. Midhurst had a lot of possession and at times were careless, with Selsey looking dangerous on the break.
Midhurst forced four consecutive corners on stroke of half-time. Hamilton’s corner was headed home by James Liddiard in stoppage time but there was another twist to come before the break. Straight from the kick off Selsey worked the ball to Max Davies, who coolly slotted past Josh Bird to send the sides in level.
The Stags came out in the second half and caught Selsey out in the 47th minute with Laurie Smith playing a ball into the area, picking out Harry Tremlett who finished to restore the Stags' lead.
Set-pieces were causing Selsey more problems and another corner wasn't dealt with and Harry Giles was there to add a fourth in the 66th minute. Bedford got his second in the 73rd minute with a smart finish.
Stags boss Andy Ewen said: "I wasn't happy at the break - we were slow and lethargic and our game management was poor. We twice conceded goals within a minute of taking the lead. Second half we moved the ball quicker and looked dangerous, it’s another win and we’ll take it."
Selsey boss Daren Pearce said: "It was not our day despite drawing back level just on half time at 2-2. The lads let themselves down second half conceding three sloppy goals and losing, giving Midhurst the three points on the day. There are no complaints from us - we weren't good enough on the day.
"We go to Wick tonight (Tues 24) which is a tough one, then have Hailsham at home on Saturday and we play high-flying Roffey, who have added a lot of quality players from the level above to their side - they visit us on the Tuesday night.
"There are some tough games - they won't define our season but if we want to be among the top half of the table then we need be picking up points in the next three games."