On disconnect the chat is completely cleared. If I haven't been watching the chat just as I disconnect I don't know what has been said right before the disconnect. I understand obviously that I won't be able to see what people write when I am disconnected.

What I am wondering is if it's possible to avoid clearing the chat log upon disconnect? Like, just connect right back in with the previous chat still there. Watching the logs is a hassle everything I disconnect, which I do quite often, for reasons related to my ISP.

mIRC Options -> IRC -> Logging -> Reload Logs -> Both or Channel

As an alternative to reloading logs, you may want to try the Keep channels open option which keeps the channel windows open when you get disconnected.

options (alt+r) > irc > select Keep channels open