If, when, and how other terrestrial planets were (or are) habitable is one of the primary foci of planetary science. The catalog of known planets around other stars has exploded in recent years, and there is more data than ever before that might be usable to determine whether any of these exoplanets are habitable or inhabited. The habitability of a planet through time -- where habitable conditions are defined as those enabling surface liquid water to be present on the planet's surface -- is the product of many interconnected processes spanning the core to the upper atmosphere. The abundance and resolution of data available for Mars and Venus make these the only planets besides the Earth where we can identify and interpret the geologic imprints of these processes and their resulting climates. In this thesis, I use Mars topographic data, measurements of Venus' atmospheric composition, statistical analysis, and numerical modeling to integrate proxies for past conditions on both planets into our understanding of their overall evolution. In the first part of this thesis, I focus on the climate record provided by martian geomorphology. In Chapter 2, I compare the size distribution of ancient buried craters to impact model results to constrain atmospheric pressure during one of the wettest eras in Mars' history. In Chapters 3 and 4, I use measurements and models of valleys exiting small, relatively young craters with overspill channels but no visible inlets to constrain the volume of water needed to explain their formation under a range of climate conditions, and infer that Mars' climate has been ice-dominated but transiently and locally habitable for billions of years. In the second part of this thesis, I turn to the evolution of Venus as constrained by its modern atmospheric composition. I present models of bulk atmospheric composition and radiogenic argon degassing which I use to explore the narrow range of parameter space that enables a past habitable Venus to be reconciled with the planet's present, and make recommendations for future spacecraft observations that may reveal the likelihood of clement past conditions on Earth's sister planet.