The gender dynamics of political discussion are important. These dynamics shape who shares their political views and how they share their views and reactions to these views. Using representative survey data from the United States and the UK, we investigate how social media platforms shape the gender dynamics of political posting. We find that on Facebook, gender does not predict political posting, whereas on Twitter, the gender gap is more pronounced. We also examine the concept of “mansplaining”—a term used to describe a patronizing form of communication directed at women by men. Firstly, we find that posting about political issues to Twitter is more likely to result in being an explainee but also being an explainer of political issues. Furthermore, posting to Twitter increases the likelihood of men reporting having been accused of mansplaining and women reporting having experienced it. In general, more than half of the women say they have experienced mansplaining, especially those who are younger, well educated, and left-leaning. We argue that the possibility of being mansplained affects who is willing to post their opinions online, and as such, caution should be exercised when using digital trace data to represent public opinion.