This can be done from within the Steam client
There is an option within Steam that toggles whether or not Steam starts with your computer.
To change it, open up the main Steam window and select 'Preferences' from the 'Steam' menu, followed by selecting the 'Interface' tab, which brings up the following screen; Simply uncheck 'Run Steam when my computer starts' to prevent Steam from starting with your computer.
(Obviously if you wanted to Steam to start with your PC, you'd check this box instead of unchecking it!)
macOS
If, on macOS, this doesn't work for you, check for Steam in "System Preferences" > "Users & Groups" >
[Your user]
>
"Login Items", and remove it via the '-' sign.
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An alternative solution on Windows (starting from 8) is to open task manager and disable Steam Client Bootstrapper from the Start-up tab.
Note:
This is an override switch to Steam's setting.
You cannot re-enable it from Steam settings after disabling it here.
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In Windows 7 click the Start menu and type msconfig in the Search box.
In the System Configuration program that opens, click the Startup tab, and then uncheck Steam.
This does pretty much same thing as the Windows 8 tip.
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OSX
= open steam then next to apple icon in top left cornersteam > preferences >
interfacethen uncheck - 'run steam when my computer starts'
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For Mac users: Go to the  Apple menu and choose “System Preferences”, then go to “Users & Groups” Select the user that is active in OS X, then choose the “Login Items” tab Select “Steam” from this list and then hit the Delete key on the keyboard to remove steam from the automatic launch on login list Close out of System Preferences
gaming.stackexchange.com/users/173659/Trevor Wood
Every time I boot into OSX or Windows, Steam automatically starts.  Is there a way to stop Steam starting whenever I boot up my computer?
How can I stop Steam from loading when I start my computer?
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<steam>
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You can do it from within Steam client. You can also disable Steam Client Bootstrapper from the Start-up tab.  It is possible to stop Steam client from starting up during boot, in any operating system, withing the client itself or by commanding to do so via operating system settings.
It is stated that Steam client offers that option. While there is a way to disable Steam from starting up at boot, within Windows settings. A user guide, regarding disabling Steam starting up at boot, for Mac users.