According to ASHRAE 90.1, Is it mandatory to install occuppant sensors in electrical and mechanical rooms?

No they are listed among the exceptions to section 9.4.1.2a.

The copy of the standard that I have reads as follows: Exceptions to 9.4.1.2a: a. Lights in corridors, electrical/mechanical rooms, public lobbies, restrooms, stairways, and storage rooms. Reading further. Section 9.4.1.2b lists the spaces where occupancy sensors or a timer switch are required and electrical/mechanical rooms are not on the list. You then go to Section 9.4.1.2c which states that a control device can be manual (i.e. a light switch). Therefore such automatic controls are not required in electrical/mechanical rooms.

Hi Marcus, based on this thread, "Section 9.4.1.2b lists the spaces where occupancy sensors or a timer switch are required and electrical/mechanical rooms are not on the list" - Are occupancy sensors or any lighting control mandatory to be installed in regularly occupied spaces?

Yes. See section 9.4.1.1 through 9.4.1.7