There are times when you'll need to know how to find the serial number for your 90 HP Mercury outboard motor. One is during the titling and registration process for states that require you to title and register your outboard motor separately from your boat. Another is when you are ordering parts; knowing your serial number and supplying it to the parts vendor means that you will receive the proper parts for your motor model.
Press down on the rear latch of the top cover. Lift the rear edge of the cover and unhook the front of the cover.
Look at the top of the power head for a metalized tag embossed with the Mercury logo, the date of manufacture, the model number and the serial number. If the tag is not in evidence on top of the power head, look on the lower-starboard side of the power head.
Move to the right and look outside the motor if the metalized tag on the power head is not readily visible. Another metalized tag, embossed with with the same information, is located on the starboard side of the motor's swivel bracket.