A green, rapid, and cost-effective probe for the precise colorimetric detection of Mn(II) ions has been investigated. The AgNPs were prepared via heating method by utilizing the extract obtained from Bhilwa (Semecarpus anacardium Linn) nuts (B−AgNPs). The Mn(II) ions induce the aggregation of B−AgNPs, resulting in color changes from yellowish brown to dark red along with the red shift in the surface plasmon resonance (SPR) peak from 404 to 432 nm. The aggregation of B−AgNPs was further confirmed by FTIR, HRTEM, and DLS techniques. The developed probe displayed an excellent linear response towards Mn(II) ions in the linear range from 20 to 0.001 ppm. In comparison to the currently available methods for the detection of Mn(II), the proposed probe provides an enhanced detection limit, i.e., 0.001 ppm. The developed colorimetric probe can be successfully applied for the detection of Mn(II) ions in water samples (pond water), thus demonstrating its potential use in field applications.