Sandra is an agent of the Secret Service.
This is not the Secret Service as we know it, formerly of the Treasury Department and now a part of Homeland Security, but rather a catch-all name for some intelligence bureau in the American alphabet soup group. Which one is never revealed but it, as is to be expected, has its headquarters in Washington, D.C.
Before she became involved in international intrigue, Sandra was a well-to-do single woman living quite happily in New York City (assumed, not certain). We know she was well-off by inference in that when first we meet her, she is completing a shopping trip and climbing into a limousine driven by James (who else) and she has a butler named Haskins. We conclude she is single by the fact that when adventure calls, she takes off with no hesitation and no need to let anyone (like a spouse) know.
We know that Sandra is athletic just by looking at her pictures and seeing how she handles herself. We also know she is quick-witted and resourceful and very brave by the fact that when she is accosted by gunman, she immediately grabs a chair and hurls it at him with no hesitation. We also learn quickly that Sandra loves adventure and "excitement" as she jumps at the chance to head overseas on a mission.
As time go by, we also learn Sandra is quite capable of operating boats, using a wireless, and even piloting a small plane. How she learned all this is never mentioned but each is done with surety and skill.
First meeting Sandra, she is dark-haired. Halfway through her set of adventures, she becomes a blonde (around the time a new artist took over drawing duties).