Emerging flexible and wearable electronic systems require high-density, low-power circuits that can seamlessly integrate with soft substrates. Monolithic 3D (M3D) integration using 2D semiconductors offers a compelling route in fulfilling these requirements by enabling vertical stacking without compromising mechanical compliance. However, current M3D approaches utilizing 2D semiconductors often require high-temperature processing or transfer steps that hinder their scalability on flexible substrates. Herein, we present a low-temperature M3D integration strategy based on 2D semiconductor inks, which is enabled by a tailored anion-cation doping approach for precise carrier control in n- and p-type devices. Our methodology yields vertically assembled complementary metal-oxide-semiconductor circuits – including inverters, logic, and photosensor-integrated gates, and ring oscillators (ROs) – fabricated entirely at ≤150 °C. Notably, the inverters exhibit a voltage gain up to 462 at a supply voltage of 4 V, and the 5-stage ROs can operate at a maximum oscillation frequency of 13.5 kHz. Beyond their electrical performances, the circuits display robust mechanical stabilities, conforming to curved surfaces, and excellent skin compatibilities. This study reports a scalable, low-temperature platform for the fabrication of M3D electronics in wearable low-power neuromorphic computing systems and bio-integrated electronics.