Seaby will join Lewis Johnston as the inclusions in the Swans line-up after both were rewarded for strong recent showings in the Swans reserves.
"He's really improved over the last six months in particular," Longmire told sydneyswans.com.au.
"He's been so consistent, and he couldn't really do much more than what he was doing at reserves level."
Seaby returns to the side for the first time since being the Swans substitute in round one against Melbourne at the MCG.
"Mark's form has been building as the season's progressed, to the point that the last couple of weeks he's looked really good," Longmire said.
"He's moved really well and started taking some more contested marks ... and he's an experienced player."
Seaby will replace regular ruckman Shane Mumford who will miss this week with a sore knee, while Johnston comes in to replace Andrejs Everitt who will feature for the Swans reserves in Newcastle on Saturday.
Round 10, Sydney Swans v North Melbourne
Saturday May 28, 2:10pm, Etihad Stadium
B: N.Malceski, T.Richards, A.Johnson
HB: T.Kennelly, H.Grundy, L.Johnston
C: R.Shaw, J.Bolton, N.Smith
HF: B.McGlynn, L.Roberts-Thomson, J.Kennedy
F: J.McVeigh, A.Goodes, R.O'Keefe
Foll: M.Seaby, C.Bird, D.Hannebery
I/C: L.Jetta, M.Mattner, L.Parker, S.Reid
Emg: A.Everitt, J.Moore, J.White
In: M.Seaby , L.Johnston
Out: S.Mumford (Knee), A.Everitt
New: L.Johnston (North Adelaide (SA))
