A simple and reliable colorimetric probe N,N′-bis-(4-diethylamino-2-hydroxybenzylidene)-1,10-phenanthroline-2,9-carbohydrazide (L) has been synthesised by reacting 4-(diethylamino)salicylaldehyde with 1,10-phenanthroline-2,9-dicarbohydrazide. The sensing ability of L was studied by its interactions with various f-block metal ions and other selected metal ions from s- and d-block by colorimetry, UV-visible spectrophotometry, and smartphone integrated red-green-blue (RGB) model in DMSO : H₂O (7 : 3, v/v). The pale-yellow colour of L turns to wine-red upon interaction with uranyl ions (UO₂²⁺) and yellow-orange in the presence of Th⁴⁺, Zr⁴⁺, Fe³⁺, and Lu³⁺ ions. Other tested metal ions did not show any colour change of L. This color change offered a simple, quick, and consistent method for the selective and sensitive visual detection of trace levels of UO₂²⁺ ions without any need for sophisticated instruments. Sensor L exhibits two absorption bands at 358 and 389 nm due to ligand-to-ligand charge transfer (LLCT). Upon interaction of L with UO₂²⁺ and Th⁴⁺ ions, absorption bands are exhibited at 480 nm and 422 nm, respectively, due to ligand-to-metal charge transfer (LMCT). The UV-vis spectral studies indicated the formation of a 1 : 2 ligand-to-metal complex between L and UO₂²⁺ with an estimated association constant of 1.0 × 10⁴ M⁻². Using L, the concentration of UO₂²⁺ can be detected as low as 73 nM and 150 nM by spectrophotometry and RGB methods, respectively, without any interference from other tested ions with an RSD < 5% (n = 3). The binding mechanism was studied by ¹H NMR titration, ESI mass, and FT-IR spectral analysis and was well supported by theoretical results. Overall, sensor L demonstrates promising analytical applicability for the detection of UO₂²⁺ ions in a semi-aqueous medium.