If I hold cash in my investment account, will it gain interest?

Yes!

Interactive Brokers (IB) currently pays a very competitive interest rate on cash balances in excess of $10K, as long as your total account value is over $100K. Interest accrues and is payable on a daily basis, and IB posts actual interest monthly on the third business day of the following month. Please visit IB’s website for the most up-to-date information on interest rates.

Charles Schwab also pays an interest rate for uninvested cash held in brokerage and retirement accounts. Please visit Schwab’s website for additional information about the current interest rate for your Everyday Cash balance.