The former Lincoln City duo were on target at different ends of the second half as the Pilgrims made it three wins from three in the last week with victory at Altrincham.
Marriott's 49th minute opener was supplemented by Arnold's 89th minute strike as United bossed proceedings at the J Davidson Stadium.
Chances were at a premium in the first half although Arnold did sail an early effort wide from 22 yards while Nicky Walker, starting in attack for the first time this season, rattled the post two minutes before half-time following a slick United move.
Walker had been handed a start, while Toby Lees was included in the starting XI for the first time with Ryan Qualter missing through injury.
United took the lead four minutes after half-time when MARRIOTT crashed home a low 20-yarder after working himself some space outside of the Altrincham area.
The home side introduced Port Vale loanee Brendon Daniels from the bench and his long-range free-kick was repelled by the United defensive wall.
The Pilgrims went close to killing the contest off when Ben Middleton headed narrowly wide on the end of Arnold’s corner.
Altrincham remained in the contest and Daniels sent another drive narrowly wide from 20 yards.
United did not need to be too concerned though and sealed the points with a minute to play when substitute Craig Westcarr slipped ARNOLD into the clear and he beat Tony Thompson in the Robins’ net.
Altrincham (4-2-3-1): Tony Thompson; Andy White, Liam Goulding, Tom Hannigan, Connor Hampson (Sean Williams 66); Jake Moult, Max Harrop (Brendon Daniels 62); John Johnston (Tom Peers 73), Simon Richman, Josh Hancock; Jordan Hulme. Subs (not used): James Jones, Elliot Wynne (gk).
Boston (4-4-2): George Willis; Ben Middleton, Toby Lees, Spencer Harris, Ashley Jackson; Nathan Arnold (Jordan Gough 90), Brad Abbott, Andi Thanoj, Jonathan Wafula; Adam Marriott (Craig Westcarr 73), Nicky Walker (Grant Roberts 89). Subs (not used): Jonny Margetts, Andre Johnson.
Referee: Matt Corlett.
Attendance: 1,067.