The Magnetic Plage Strength Index (MPSI) and the Mount Wilson Sunspot Index (MWSI), which have been measured at Mount Wilson Observatory (MWO) since the 1970s and which indicate weak and strong magnetic field activity on the solar full disk, respectively, are used to systematically investigate midterm periodicities in the solar full-disk magnetic fields. Multitudinous mid-term periodicities are detected in MPSI and MWSI on timescales of 0.3 to 4.5 yr, and these periodicities are found to fluctuate around several typical periodicities within a small amplitude in different solar cycles or phases. The periodicity of 3.44 yr is found in MPSI, and the periodicities of 3.85 and 3.00 yr are detected in MWSI. Our analysis indicates that they reflect the true oscillating signals of solar magnetic field activity. The typical periodicities are 2.8, 2.3 and 1.8 yr in MPSI and MWSI, and possible mechanisms for these periodicities are discussed. A 1.3 yr periodicity is only detected in MPSI, and should be related to meridional flows on the solar surface. The typical annual periodicity of MPSI and MWSI is 1.07 yr, which is not derived from the annual variation of Earth's heliolatitude. Several periodicities shorter than 1 yr found in MPSI and MWSI are considered to be Rieger-type periodicities.