A below-par first 45 minutes left Rovers trailing at the break in their most competitive test yet, but goals from Liam Ravenhill, George Miller and Ro-Shaun Williams capped off a much-improved second period.
Spennymoor took the lead after eight minutes when central defender Joe Oluwu misjudged the flight of the ball, which sparked a counter attack finished by Mark Anderson, whose volley flew past Jonny Mitchell and into the top corner.
Much of Doncaster’s best work in the opening half came through ‘Trialist B’, a young midfielder who was hooked at half-time before being reintroduced for the final 15 minutes.
He was involved in their best chance when a one-two released ‘Trialist A’, a left-back – Cieran Dunne was not involved – but his cross was volleyed over by Reo Griffiths on the stretch.
Doncaster also lost captain Adam Clayton to injury after 30 minutes, with Harrison Biggins taking his place in the centre of midfield for his first appearance since joining from Fleetwood.
A triple change at the break saw Miller, Charlie Seaman and Molyneux replace Griffiths, Ollie Younger and Trialist B – and it quickly paid dividens.
Molyneux, also making his first appearance, brought with him some much-needed creativity from the off.
Four minutes after the restart he cut inside from his favoured right-ring position and beat several defenders before picking out Liam Ravenhill with a low pass, who made no mistake from ten yards.
With Rovers increasingly in control and looking far more fluent going forward, Miller made it 2-1 from the penalty spot with his third goal in as many games after Lee Tomlin was brought down by Spennymoor keeper Harry Flatters.
Five minutes later Ro-Shaun Williams scored their third with a looping header at the far post from Molyneux’s corner.
Doncaster continue their preparations for the new campaign at home to Huddersfield Town on Saturday.
Doncaster: Jonny Mitchell (Louis Jones; 65), Trialist A (Jak Whiting; 71), Joe Oluwu (Bobby Faulkner; 56), Ro-Shaun Williams, Adam Clayton (captain) (Harrison Biggins; 31), Tommy Rowe (Trialist B; 78), Ollie Younger (Charlie Seaman; 45), Trialist B (Luke Molyneux; 45), Reo Griffiths (George Miller; 45), Lee Tomlin (Kieran Agard; 67), Liam Ravenhill (Jack Degruchy; 71)