This study uses WhatsApp to facilitate online discussions in developing content idea for ESL writing. A quasi-experimental study was conducted with 33 pre-university students participating in online discussions on WhatsApp for five weeks to support face-to-face instructions. Pre-test, post-test, and five weekly writing tests were administered, examined, and analysed to identify the participants' writing performance. The findings show significant improvement in the experimental group's essay writing scores after using WhatsApp for online discussions. The participants were able to use the content generated from the MWEG in supporting the arguments in their essays. Besides content idea development, the experimental group saw the WhatsApp online discussions as boosting confidence, raising motivation, and encouraging interactions among students. The themes that emerged from the WhatsApp group interaction were lively discussions, sharing of opinions, and turn-taking. While the control group also improved in their test scores, it was not as significant as the experimental group.