Login, spelled as one word, is only a noun or an adjective. For example, the information you use to sign into your email is your login (noun), and the page where you sign in is the login page (adjective). Log in is two words when it functions as a verb. For example, you log in with your login information.
There is much precedent for the distinction. Many two-word phrasal verbs have one-word equivalents that function as nouns and adjectives—for example, check up and checkup, pay back and payback, and run away and runaway. In these cases and the many others like them, the one-word forms never become verbs, and there is no good reason to make an exception for login.
Some spell the one-word form with a hyphen—log-in—especially in the U.S. This will likely change, though influential publications such as the New York Times have so far resisted going along with the rest of the world (including most Americans) on the spelling of tech terms (see also the Times’s use of Internet, Web site, and e-mail where most people in the English-speaking world use internet, website, and email.)
Login (noun and adjective]
At the moment the login is through Facebook, but the company plans to change that and to release an Android app. [News24]
The firm sent out a “large number” of emails this morning requiring members to change their login details. [Telegraph]
In this case, the devices are more secure, since no data is stored on the device and access will require a secure account login. [New Zealand Herald]
Log in (verb)
Users log in and are presented with a selection of plays, concerts and other events for that night only. [Wall Street Journal]
And customers can log in themselves to view all orders, invoices and payments in real time. [Guardian]
In other words, it provides a secure way for employees to log in and access all the applications. [Forbes]
These editions come with faster processors and separate log-in accounts so multiple children can share the same device. [New York Times]
[P]references in the Modern layout were automatically saved and transferred to multiple machines with a single log-in. [Denver Post]