Who invented first motherboard?

IBM, in 1981.

There was no specific invention or inventor of a first motherboard. The concept of a motherboard evolved over a number of years responding to the reduction in electronic component size. One of the original concepts in computer construction was the backplane. The backplane allowed for circuit card to be plugged in on one side. Its back had rows of pins corresponding to the socket connections. The pin connections defined how the attached circuit cards functioned and pins were connected to other pins with wires wrapped around them.

As technology matured, electronic parts were combined into Integrated circuit (IC) chips which were mounted on the backplane itself and the pins and wiring on the rear of the backplane were replaced by printed circuits. This eventually became what we call a motherboard, mainboard or some equivalent name.

Original microcomputers and personal computers had a central processor (CPU), memory, and control circuitry on the motherboard. That took up all the available space. Most other parts required their own support circuitry that would not fit on the motherboard so they were placed on individual circuit cards plugged into the motherboard. This included additional memory, output display, hard drives controllers, and even standard input/output ports such as serial (RS232,) Printer(parallel), mouse and game ports. Most of these eventually became compact enough to be integrated onto the motherboard board we use today.