Controlling ferromagnetism by an external electric field has been a great challenge in materials physics, for example towards the development of low-power-consumption spintronics devices. To achieve an efficient mutual control of electricity and magnetism, the use of multiferroics--materials that show both ferroelectric and ferromagnetic/antiferromagnetic order--is one of the most promising approaches. Here, we show that GdFeO(3), one of the most orthodox perovskite oxides, is not only a weak ferromagnet but also possesses a ferroelectric ground state, in which the ferroelectric polarization is generated by the striction through the exchange interaction between the Gd and Fe spins. Furthermore, in this compound, ferroelectric polarization and magnetization are successfully controlled by magnetic and electric fields, respectively. This unprecedented mutual controllability of electricity and magnetism is attributed to the unique feature of composite domain wall clamping of the respective domain walls for electric and magnetic orders. This domain wall feature generally determines the efficiency of the mutual controllability and thus could have an important role towards the application of multiferroics to practical devices.