Electing to bat after winning the toss, Himachal produced a solid batting performance as Bains and Akash Vasisht (30), who was making his first-class debut, laid the platform with a 87-run parnership first.
The Wicket-keeper batsman then added an unconquered 180-run partnership for the third wicket with Paras Dogra (76) to frustrated the Assam bowlers.
Bains blasted 12 boundaries and one six in his 282-ball innings, while Dogra had seven hits to the fence beside a six in his unbeaten 131-ball innings so far.
Himachal lost their first wicket in the 35th over when Vasisht was caught behind by Saikia off Ahmed, while Ankit Kalsi was dismissed by Roy in the 44th over.
Brief Score:
Himachal Pradesh 1st innings: 282-2 in 90 overs (AK Bains 156 not out; SA Roy 1/19).