This paper analyses the risk-adjusted return differences between SACCOs and MMFs in Kenya between 2019–2023. The analysis was conducted by constructing an optimized investment portfolio using the principles of Modern Portfolio Theory, which include Tier One SACCOs and registered MMFs. The evaluation of each asset class performance was done using fixed-effects panel regression, while Sharpe Ratios and portfolio optimization through simulation was used for further comparison. The results from the regression analysis confirmed that SACCOs demonstrated a positive and significant relationship between returns and deposits, while higher NPL ratios negatively impacted investor preference. Similarly, MMFs demonstrated a positive and significant relationship between annualised yield and AUM, while higher risk-weighted portfolios reduce investor participation. SACCOs outperformed MMFs in terms of risk-adjusted returns in the Sharpe Ratio analysis, validating H₁ hypothesis, that there is significant difference in risk, returns between investments in SACCO and MMF. The most efficient portfolio combination was determined using the Mean variance optimization model with portfolio allocation of 52% SACCOs and 48% MMFs, 16.66% expected return, 3.28% standard deviation, and 2.58 as the highest Sharpe Ratio. Combining the two investment options SACCO and MMF created a diversified portfolio that delivered higher returns with moderate risk.