In situ forming implants (ISIs) are popular due to their ease of use and local drug delivery potential, but they suffer from high initial drug burst, and release behavior is tied closely to solvent exchange and polymer properties. Additionally, such systems are traditionally viewed purely as drug delivery devices rather than potential scaffold materials due to their poor mechanical properties and minimal porosity. The aim of this research was to develop an injectable ISI with drug release, mechanical, and microstructural properties controlled by micro- and nanoparticle additives. First, an injectable ISI was developed with appropriate drug release kinetics for orthopedic applications. Poly(β-amino ester) (PBAE) microparticles were loaded with simvastatin or clodronate, and their loading efficiency and drug retention after washing was quantified. Drug-loaded PBAE microparticles and hydroxyapatite (HA) microparticles were added to a poly(lactic-co-glycolic acid) (PLGA)–based ISI. By loading simvastatin into PBAE microparticles, release was extended from 10 days to 30 days, and burst was reduced from 81% to 39%. Clodronate burst was reduced after addition of HA, but was unaffected by PBAE loading. Scaffold mass and porosity fluctuated as the scaffolds swelled and then degraded over 40 days. Next, the mechanical properties of these composite ISIs were quantified. Both micro- and nanoparticulate HA as well as PBAE microparticle content were varied. Increasing HA content generally improved compressive strength and modulus, with a plateau occurring at 30% nano-HA. Injectability remained clinically acceptable for up to 10% w/w PBAE microparticles. Ex vivo injections into trabecular bone improved both strength and modulus. Lastly, HA-free ISIs were investigated for drug delivery into the gingiva to treat periodontitis. Doxycycline and simvastatin were co-delivered, with delivery of doxycycline over 1 week accompanied by simvastatin release over 30 days. PBAE-containing ISIs exhibited higher initial and progressive porosity and accessible volume than PBAE-free ISIs over the course of degradation. Additionally, PBAE-containing ISIs provided superior tissue retention within a simulated periodontal pocket. The ISIs investigated here have a wide range of potential applications due to their flexible material and drug release properties, which can be controlled by both the chemistry and concentration of various particulate additives.