Old Collegians also prospered, passing 200 but fell 59 runs short.
Preston's hundred ended a run drought for St Albans players in four seasons at either one-day or two-day level. He faced 160 balls and stroked 17 fours, sharing a telling stand of 188 for the second wicket with Guy Bromley in 126 minutes.
While Old Collegians batted with considerable consistency it could not break the shackles imposed by Canterbury slow bowler Mark Priest whose 10-over stint yielded just eight runs with three maidens.
Defending champion Riccarton got home by the day's narrowest margin, 13 runs against Sydenham at Sydenham Park.
Riccarton, without batting mainstays Gary Stead and Robbie Frew in the Canterbury team, reached 197 for nine with Aftab Habib top scoring.
Sydenham produced an improved batting performance on its recent efforts, Kieran White leading the way with 40, while Canterbury rugby player Leon MacDonald swapped codes to good effect, making 20.
A fine innings of 81 from Robert Tibbotts underpinned Lancaster Park-Woolston's 205 for eight and a 42-run win against East-Christchurch Shirley at Garrick Park.
Tibbotts and Mark Lane batted productively, while East-Shirley succumbed for 163 with only Michael Papps threatening to dominate.
High School Old Boys completed a good day for the team's batting first by beating Burnside West-University by 23 runs at Burnside Park.
Old Boys began well reaching 94 for one with Jeremy Innes striking a half century and batting well in tandem with James Barrett. But Hamish Kember helped apply the clamps, taking three for five, although strangely he only bowled eight of his 10 allowable overs. Burnside lost wickets at regular intervals and struggled to threaten the target.
Scores
HSOB 151 (J Innes 50, J Barrett 24, S Martin 21; H Kember 3-5, D
Maxwell 3-27, C Ogilvie 3-33) beat
Burnside-West 128 (S Blackman 31no; S Martin 3-12, J Barrett 3-40, H
Wardrop 2-26) by 23 runs.
St Albans 274-3 (R Preston 109, G Bromley 78no, extras 46) beat
Old Collegians 215 (D McCarthy 45, M Hudson 30, A Kininmonth 27; M
Griffen 3-33, G Charles 3-44, J Ward 2-46) by 59 runs.
Riccarton 197-9 (A Habib 34, extras 33, G Foulkes 28, W Stead 28, C
Bray 25; S Hartill 3-42, G O'Reilly 2-32) beat
Sydenham 184 (K White 40, B Riori 24, C Wood 21, L MacDonald 20; D
Walker 2-33, L Borcoski 2-31, G Foulkes 2-26) by 13 runs.
Lancaster Park-Woolston 205-8 (R Tibbotts 81, M Lane 39; S Flanagan
2-39, J Pawson 2-41) beat
East-Shirley 163 (M Papps 49, D Grocott 27; J Quinn 2-9, A Richardson
2-21) by 42 runs.