The 18-year-old striker, who was playing boys' club football last year before he signed for the Edinburgh club in the summer, hit a 25-yard drive past Kevin Cuthbert in the 39th minute to give the visitors the interval lead in the first leg of the play-off final at New Douglas Park.
Accies were rocked again seven minutes into the second half when former Hearts youth player Cummings grabbed a second from much closer range.
Only a remarkable turnaround in the return at Easter Road on Sunday would see Hibs lose the final.
A large and boisterous travelling support made their way to the compact Lanarkshire ground, with its artificial surface arguably giving Accies, who finished second to Dundee in the Championship, another advantage.
However that did not appear to be the case in the early stages as Hibs, despite having won only one of their previous 19 games, looked confident and they were unlucky in the 12th minute when midfielder Scott Robinson's drive from 20 yards struck the outside of the post.
The visiting defence, though, looked nervy any time a cross came into their box which offered the Accies support encouragement.
Hamilton were lively going forward but on a couple of occasions failed with the final pass which could have had the visiting side in real trouble.
The breakthrough almost arrived in the 37th minute when Accies midfielder Louis Longridge drove a shot just wide of the junction of crossbar and post from 12 yards, after being set up Darian MacKinnon.
However two minutes later Cummings changed the complexion of the tie when he unleashed a ferocious drive from 25 yards which had Cuthbert grasping at air.
It was a timely reminder to the Leith fans in the crowd of 5,322 that their side were more than capable of shooting themselves in the foot.
However, when Cummings scored his second just after the break it suddenly became a long way back for the home side.
Paul Heffernan's driven cross from the right ended up with the teenager and he took a touch before dinking the ball over Cuthbert, and although Hamilton defender Ziggy Gordon cleared the ball had gone over the line.
Accies were stunned and were left with no option but to throw themselves forward in the hope of pulling a goal back.
The home side kept at it and in the 74th minute Williams had to come smartly from his goal to smother an angled shot from substitute Andy Ryan, on for James Keatings.
Hibs remained mostly in control until the final whistle, with the Accies players sloping off knowing they need to put in the performance of their careers come Sunday if they are to be in the top flight next season.
- Sports & Recreation
- Kevin Cuthbert