The emergence of generative artificial intelligence (GenAI) has caused significant disruptions on a global scale in various workplace settings, including the field of instructional design (ID). Given the paucity of research investigating the impact of GenAI on ID work, we conducted a mixed methods study to understand instructional designers (IDs)' perceptions and experiences of utilizing GenAI across a spectrum of ID tasks. A total of 70 IDs completed an online survey, and 13 of them participated in the semi-structured interviews. The survey results indicated IDs' familiarity with and perceived usability of GenAI tools in performing various ID responsibilities in their specific contexts. Qualitative findings further explained that IDs often utilized GenAI tools in (1) brainstorming ideas, (2) handling low-stake tasks, (3) streamlining design process, and (4) enhancing collaborations. Participants also expressed their concerns and challenges while using GenAI in ID, including (1) quality concerns, (2) data security and privacy concerns, (3) concerns over authorship, ownership and plagiarism, amongst others. Implications and recommendations are also discussed to inform future ID practices and research.